As one of the people interviewed for background said, there is often no connection between the hotel computer and the key-lock on the door. I think this is mainly on older installations, but I noticed when I had a problem with my key lock at the Tropicana this year and the security man came out to investigate that there are no wires connecting to the key lock. It is battery powered (and the battery in mine had gone dead). This means that at least in older hotels, if you keep your card key, and go back a few days later, you can probably gain entry to the same room as they have no way to associate a new code with the lock.

The security princlple is that there is no number marked on the card, so no one could use it to gain entry. It appears a former occupant who kept the key could, though.