The keys approach suggested here would actually be a good way of implementing it. Either that or it's just been suggested a lot more clearly then before I would suggest that if the owner of the keys gets into another vehicle he would lose the lock on the first vehicle. And perhaps some sort of graphical notification on the locked vehicle so you're not wondering why you can't get in. The timer would have to be kept low to avoid griefers. Maybe 30 seconds or a minute at most?