The smart key is just that... smart! You CANNOT lock the key in the trunk or the car itself. You have to either be holding the key or it must be somewhere on you (pocket or purse) in order to lock a door. If you leave the key in the car and then shut the door, the car will do a long beep at you when you go try to lock the door. It will not lock you out! Drop the smart key in the trunk and shut the trunk door, it will again do a long beep and cannot lock because you left the smart key in the trunk which you need to lock the trunk with in the first place. Thanks for that because I have dumped my purse in the trunk while putting the groceries in, shut the trunk door only to have my car "yell" at me because I left the key in there! It's nice to know I'll never lock myself out of the car leaving the key behind!