Not trying to be acquisitory, but I too find it unlikely that the RFID card would open even one other car. While I've never seen a technical write-up on the key cards, I doubt Tesla is using the RFID card's ID number. If they are, then conceivably someone could duplicate your key by simply scanning it via brushing up against your wallet. My guess would be that the authentication is a cryptographic challenge and response sequence where the RFID card's hard coded key is never revealed on the radio channel. That said, there have been many past examples where companies have taken the easy route over the secure route.