Right ive pinned it down to what was unlocked.
There is a mission called cops and crooks (star icon on the map) where you have to scan various cars, in which sit the other team. you cant see their blip until you scan them. Its set in a small area of the city, and is like a TDM but with car's.
I got it around lvl 10 but from talking to my guild, they didnt get it so it might be based on story missions (since ive been grinding them out).
SO as mentioned above, its NOT cops and crooks mode, but instead a mission called cops and crooks.
Now, as for the buffalo, not seen the marked car. But the FBI cars still spawn at 11am on a monday at the satellites however they too cannot be saved in the garage. But if you want the unmarked buffalo, you can get it there.
As it is,ive given up trying to do any form of cop RP, since i cant be bothered with the wanted level everytime someone see's me in the car. Especially since it takes ages to get anything but the interceptor. Its just not worth the time.