One tip I learned from driving my F250 Crew Cab Diesel pickup for 100,000 miles, if you have something on the back of your car (or in the bed of a really big truck) back into whereever you park. Back close enough to a wall or car behind you so the tail gate can't be opened or a bike can't be easily removed from a rear rack.

Another car can leave so that's why you should try and back it all the way up against a wall, post, etc. Won't make the bike impossible to steal but makes it a less easy target.