The unlock I have not encountered.. Although there have been multiple times where I have had my car unlocked and stood by it for about 3-5 minutes and depending on how far my key gets away from the car the doors will automatically lock; therefore you can't ever forget to lock your car.
Now if someone gets in your car and takes off within that 3-5 minutes you are gone from your car (and not testing like myself) then well.. you are SOL.