Longtime lurker, first time poster but I found this loophole on accident and though others would like to know.
Start second hunting side mission with Cletus called Fair Game. After killing 3 elk, Cletus takes off you're supposed to kill your last elk. But instead just go explore whatever area of the map you want. There are almost no other humans around and all the restricted area alarms don't go off. I was able to just drive into Fort Zacundo with no stars and take the Lazer jet. Nobody tried to come after me and I flew around and parked the jet back at my hanger. Eventually I failed the mission because Trevor died.
WHAT YOU CAN DO
Go anywhere on the map including restricted areas. (when flying the warning for restricted airspace comes up but no stars follow)
Kill anyone including military, police and other NPCs.
Steal any vehicle you want including Lazer fighter jet, Rhino tank, or Titan supply plane.
WHAT YOU CAN NOT DO
Make A LOT of noise. Explosions will be heard by the elk even across the map and you will fail the mission.
HOW TO KEEP YOUR VEHICLE
Make an explosion so you fail the mission and choose not to restart mission, Now you are free to move it to anywhere on the map.
Titan doesn't fit in any hangers.
Rhino doesn't fit in some garages.
Prison has nothing interesting.