This topic is about a recently observed mechanic on the PSP version of the game regarding marked vehicles and their attributes, as well as their applications.
What are they?
Under normal circumstances, one can leave up to two vehicles in memory by simply entering them and preferably leaving the driver's door open when going away. Once the player enters a third non-mission vehicle, the "older" vehicle left in memory will vanish when out of sight.
Now, certain vehicles make use of a different part of the game's memory. These vehicles are referred to as mission-important, owned by script, or "marked", as we commonly call them. Recently, a phenomenon was discovered which allows you to fully own vehicles that behave as if they were mission-important, whereas they're not. Their most notable characteristic is that they never disappear, unless you somehow unmark or destroy them (however, wrecks of marked bikes don't disappear for some reason), or save and reload the game with them outside the garage. Plus, you can have as many of them scattered around the map as you like, and more. You can leave as many as 12 at a time, and still have two regular vehicles in memory if you want to for example, but the game starts to behave erratically if too many are loaded. For more details, keep reading. Also, if you are Russian, you might want to check this video by zazaza691 out, which explains all about marked vehicles in detail:
Attributes of marked vehicles
Immune to "CLEAR_AREA" commands: As mentioned above, since they use a different part of the memory, marked vehicles don't vanish, even being unaffected by "CLEAR_AREA" commands, usually called during missions.
Wasted/Busted is not a factor: That's right. If you respawn outside the Hospital/Police Station after getting wasted/busted, the marked vehicle will remain where you left it. As a plus, if the cops bust you whilst driving your marked vehicle, it will be waiting for you in the exact place you got busted.
Criminals don't attempt to steal them: Now, thieves attempt to steal a wide range of vehicles from the player, or even NPCs, but there are some exceptions; mostly mission-important and emergency vehicles (Rhino, Ambulance etc.), as well as bikes they were not riding in the first place. Marked vehicles are also included in the exception list, meaning that thieves will never attempt to steal one, even if it is a model they would otherwise go for, like the Cheetah.
Ability to teleport inside with both doors blocked off: If you block both side doors of a marked vehicle by parking it in a very tight spot, or just between two cars, you can teleport inside it as soon as you touch it and press Δ.
Respawn back on land when falling through grey hell: If a vehicle somehow glitches through grey hell and you're not the driver, it will normally disappear. However, if it is marked, it will respawn back on the nearest path of land after a while. That said, if a vehicle also possesses the Heavy property and is thrown in the water, since Heavy vehicles usually sink instead of floating, thus falling straight into grey hell (as seafloor is not programmed in this game), it will get teleported back on the closest driveway. Keep in mind that if you fall in the water whilst driving a Heavy marked car, your Health will start going down and you'll eventually drown, and while you'll find your vehicle waiting on the road if you go back to the place you drowned after respawning outside the Hospital, you'll be unable to enter it, and the only way to fix this is by pushing it all the way to a garage and storing it. Another important note about this case is that your vehicle partly counts as destroyed, so if you get careless in front of the garage door, it will disappear, despite its marked status.
Identifying marked vehicles
You'll know whether a vehicle is marked or not by blocking both its side doors off and attempting to enter it. If nothing happens, the vehicle in question is not marked. If you get teleported inside, it is. Teleporting inside vehicles with both doors blocked off is a consistent sign to determine if a vehicle is marked.
This phenomenon is caused by two flags: 01CB (found in the script as "remove_car_from_mission_cleanup_list", or "DONT_REMOVE_CAR" when extracted from Sanny Builder) and 0677. The former "marks" the vehicle, and the latter allows garages to process it. Without the 0677 property, marked/mission-important vehicles wouldn't be able to be stored in garages. Now, two very unique vehicles exist in the game which possess both 01CB and 0677 (the former is found on some other vehicles throughout the game, but the latter is only found on these two), namely the prize Avenger from "Avenging Angels" (Portland) and the Landstalker from "See the Sight Before Your Flight". The programmers have probably given these two properties to the aforementioned vehicles to prevent them from vanishing due to the player being too far away from their spawn location, and to allow you to keep them.
As for what happens with the garages, there is a theory that has not yet been firmly established, but observed in practice, and works without fail. When you store a vehicle in the garage, its properties (Heavy, improved handling, BP/FP/EP/DP etc.) are stored locally. However, the properties associated with marking (01CB and 0677) are stored globally, meaning that all scripts have access to them. Let's see what this means in practice. A garage can hold a total of 4 vehicles inside in the form of slots. For this example, we will use an Avenger, a PCJ-600, a Stinger and a Banshee, which are stored as follows:
Slot 1: Avenger
Slot 2: PCJ-600
Slot 3: Stinger
Slot 4: Banshee
The Avenger possesses the 01CB and 0677 properties by default (much like the Landstalker from the tourist missions). Now, we close the garage door and reopen it. As Save Editor users may know, this shuffles the order of the vehicles at random, and after opening the garage, we get this:
Slot 1: Stinger
Slot 2: Avenger
Slot 3: Banshee
Slot 4: PCJ-600
What happened here? The "marked" properties (01CB and 0677) of the Avenger are stored globally in slot 1. When we closed and reopened the garage door, the Avenger was moved to slot 2, but is still marked, and the Stinger that moved to Slot 1 is marked too! So what we did was essentially "spread the marked virus". Now, the "disease" is stored in both slot 1 and slot 2, meaning that whichever vehicle is stored in either slot 1 or slot 2 after closing and reopening the garage door once again, will be marked. Likewise, we can also "infect" slots 3 and 4 by repeatedly closing and opening the garage. If we take one of our "infected" vehicles to other garages and repeat the process, we'll eventually have a Save File that can mark vehicles without fail. But wait, it doesn't stop here. The "marked virus" spreads among other Save Files as well; if I give you an "infected" Save File, you load it into your game, and then load one of your own, you'll be able to mark vehicles too!
As explained above, there are two methods to acquire the so-called "marked virus", which enables you to mark vehicles, as well as store them in the garage.
Method 1: Avenger/Landstalker - This is the most standard method, since it can infect "clean" Save Files without the need of infected ones. Simply obtain and store either the Avenger from "Avenging Angels" or the Landstalker from "See the Sight Before Your Flight" in the garage among three other vehicles, and keep opening and closing the garage door until every single one of them becomes marked. Then store one of your marked vehicles inside other garages and repeat the process until all 12 slots are infected, so you have a Save File that can mark vehicles without fail.
Method 2: Infected Save Files - If you already have at least one infected Save File, you can spread the marked virus to the rest fast and easy. Simply load the infected Save File, and then the one you wish to infect. Confirm your marked vehicles inside the garages and save the game.
There is a number of ways you can unmark vehicles, usually during missions. Some missions can actually unmark your vehicle under as of yet undetermined circumstances, thus making it prone to vanishing, as well as vulnerable to "CLEAR_AREA" commands again. Below is a list of all currently documented missions that can unmark vehicles:
Contra-Banned: As soon as you start the mission and enter your vehicle to pick up some backup, it becomes unmarked.
A Volatile Situation: Vehicle is unmarked upon driving into the marker next to the casino.
Vigilante: Your Police vehicle becomes unmarked the moment you start the side mission.
Trash Dash: Your Trashmaster becomes unmarked the moment you start the side mission.
Firefighter: Your Firetruck becomes unmarked the moment you start the side mission. As a bonus, although you wouldn't want to manually unmark your vehicle, but rather try to avoid it as much as possible, you can consistently unmark any vehicle by starting "Firefighter" inside it. This can be done by holding the side mission button (Up on PSP, R3 on PS2) down whilst inside the Firetruck, exiting it, and releasing the button as soon as you enter the desired vehicle. If you want to convert vehicles to PP using the "Firefighter" method, you should keep this in mind, as you'll have to mark your vehicle again.
Strangely, some side missions like "Taxi Driver", "Paramedic" and "Karmageddon" don't unmark your vehicle, but others like the aforementioned "Vigilante" and "Firefighter" do. It is currently unknown what are the complete requirements for a vehicle to become unmarked, so this section needs more research.
The PS2 version
As of yet, no case of marked vehicles besides the aforementioned Avenger and Landstalker has been reported outside of missions on the PS2 version of the game. According to ClaudeIzABadAzz, these two remain marked after storing them in a garage. More reports regarding the PS2 and even mobile versions are appreciated.
Applications of marked vehicles
Mission and free roam utility: Since marked vehicles don't disappear and are immune to "CLEAR_AREA" commands, you can drive them around all you want or leave them in one place without fear of getting wasted/busted or a criminal stealing them, and even use them to complete missions, although this needs a more thorough investigation, since, as mentioned above, some missions can unmark them. As a bonus, wrecks of marked bikes don't disappear for some reason, so if one gets destroyed, you can restore it in a garage.
Garage management: The Portland garage in particular poses a problem for storage; if you fit two cars inside, you have to push them out in order to take them for a drive, and this can take time, especially if they possess the Heavy property. Since you can teleport inside marked vehicles with both doors blocked off, you can use this exploit to drive your desired car out in an instant. Additionally, if you turn big vans/trucks such as the Yankee, Triad Fish Van, Mr. Wongs and Mr. Whoopee into "Ghosts" using the crusher glitch and store them inside the Portland garage, assuming the garage successfully marks them, it is possible to get them out by parking a bike in front of the door to keep it open, entering the safehouse and exiting it. The van/truck should glitch through grey hell and respawn on the street behind the garage, as shown in Lennart -'s video here:
The exact reason why this happens is currently unknown, but it most likely has to do with some parts of the map becoming unsolid when you change interiors, travel to other islands or go down the subway, and the Portland garage being one of them. Needs more investigation.
Special Vehicles: Special Vehicle collectors do face issues with garage storage capacity from time to time, especially early on in the game, often having to resort to leaving vehicles in memory until they unlock the next safehouse, which can be troublesome, as the game tends to delete vehicles stored in regular memory, even if the players do nothing wrong. However, this becomes no issue should collectors be able to mark vehicles, although big trucks that don't fit inside the Portland garage still pose a problem, since they can't be marked when needed to the most. Also, you can use the Ghost Town exploit (see below) to limit or disable traffic, and eventually spawn a No Extra (graffiti-less) Hoods Rumpo XL.
Entering the RC Bandit (PSP only): On PSP, should one manage to obtain the RC Bandit and attempt to enter it, the game will crash, as no entering and exiting animations are programmed for the toy. However, if you mark the Bandit and block both its "doors" off, you can teleport "inside" it and normally drive it around, although this exploit doesn't help with the exiting animation, so in order to exit it without the game crashing, you must once again block both its "doors" off to skip the animation, or make the game use an entirely different animation by building up speed and bailing out. On PS2 and mobile, even though no cases of marked vehicles have currently been reported, the RC Bandit is fixed, thus Toni can enter and exit it like any other car.
Bringing the Landstalker to Portland early: Some players like to do Starter Saves; completing every side quest that can possibly be done before the first or second main storyline mission. That said, it is possible to complete "See the Sight Before Your Flight", the marker of which is located in Shoreside Vale, before "Slacker". Upon completion, the player is awarded a unique Landstalker - also appearing in Shoreside Vale - possessing a multitude of properties, with Heavy and 01CB (marked) being among them, so it is possible to bring it back to Portland before formally unlocking the bridges without fear of losing it, by going to this location;
building up speed and bailing out right before you fall into the ocean. Then you can use a boat to cross the sea, and the Landstalker will be waiting for you on the other side.
Ghost Town: The more vehicles you have loaded during gameplay, the less traffic spawns. This can be exploited by marking vehicles and leaving them out of the garage until the traffic density reaches zero. If you have a total of 15 vehices in memory, marked or not (you can leave up to two vehicles in regular memory, so you have to mark at least 13), traffic will stop spawning, but this will make the game unstable, like failing to load vehicles with set spawn points such as the Pizzaboy and TOYZ (unless included in the vehicles you marked/left in memory), or crashing when attempting to start missions that involve a lot of vehicles like "Car Salesman", potentially locking you out of some activities that count towards 100% completion. Also, going down the subway if too many vehicles are loaded makes the game crash. You can bring traffic back if you delete vehicles by any means, such as unmarking them, to make the game stable again. Full details about other irregularities caused by having too many vehicles loaded are unknown at this point.
Matters in need of research
More insight on the requirements to unmark vehicles.
More reports of marked vehicles on PS2, as well as mobile versions.
Details on what happens if too many marked vehicles are loaded.
Since it's a pretty rare case, almost nothing is known about marked vehicles driven by NPCs.
Special thanks goes to Nick007J for helping with technical information (0677 in particular), as well as Lennart - and zazaza691 for assisting me with my research.
Keep in mind that this information holds true for the Liberty City Stories title of the Grand Theft Auto series (mostly PSP versions, as other platforms are in need of a more thorough research), and might not be the case for other games. For example, in Vice City Stories, you are granted the ability to teleport inside vehicles with both doors blocked off, but the vehicles in question are not necessarily marked.
Also, if you spot any typographical, syntactic, and most importantly, technical error, please let me know.