Don't know about the Model S or X, but I know for the Model 3 you are supposed to use Bluetooth and their app on your phone. For backup, there is a card that talks via NFC to a reader in one of the door pillars. I suspect that Tesla have considered the problems with existing RF keys and decided Not To Do It That Way.

During one of the later Top Gear adventures, Clarkson realized that they were snacking one morning close enough to their cars that Hammond's transponder was still in range. So when Hammond when to the bathroom, Clarkson simply re-parked Hammond's car in the middle of the nearby (quite busy) street. Hammond didn't figure out what had happened.

Wade.