When I quit a game my character disappears even if he was in a car. So my parked car can be opened and stolen. My suggestion is to add a mark (a variable to a vehicle structure) that marks the car "still contains a driver". So the town could be more realistic if there'll be parked players cars.
1. Formally the driver is sleeping inside. I could be conditionally. No vision of the driver and no possible comunication.
2. Bobby pins must be disabled for such cars (a message about it) so it forces the theafs to search for available car.
3. The cars can be towed by a tow truck if they are parked incorrectly (on mission marks, at the center of the streets, on pavements, blocking a passage) or if another car has crashed into it. In this case the driver spawns somewhere at the tow service building.
4. If the player haven't been played for [some peroid of time] his vehicle could be towed too - admin decides.
P.S. I'v got the idea when I went out from the hospitel and found 5 admin marks "do not steal a car from here". There could be normally parked cars without far-fetching...