(I also don't even usually lock my car... I live in an area with more deer than humans and don't keep anything in my car, I'm not too worried. I'd rather the thieves open my door to find out it's empty than smash in a window. But don't tell anyone!)
Well, this remember me af a friend, who worked in a day center for drugs-addicts (they went there to get free methadone).
He had an old Fiat Uno, which was maybe the easiest car in the world to open (and the most stolen car in Italy), and they opened it about twenty times.
A clever thief could open it by simply pulling out the upper side of the front door: the windows will hang out and there will be enough space for the hand to reach the door lock.
He ended up leaving a windows sign saying: "the doors are unlocked, and there's nothing to steal inside", but the druggiest did not believe it, and they smashed the window..
I once has a Ford Escort (1980's model) without remote or even centralized closure, which had a very "clever-stupid" not well documented feature: if you closed the driver's door while holding the lever up (as to open the door) the door would lock.
This was though of being a nice feature (you could close your car without using the keys) but ended in a lot of people locking the door with the keys inside the car.
Best regards