To take advantage of this, one would have to know the room number and be correct on the date of checkout. I think they usually ask at least a name on the room when re-keying at as well. But either way, someone could not just go randomly to the front desk and get a key for XXX room by asking a late checkout. Now if you knew that info about someone and his room, I could see it being a problem, but that would mean an acquaintance would be taking advantage of the situation. Seems a very small risk compared to when a room is being service. See something valuable (laptop, ipod) through the opened door, flash a room key, say you forgot something, and off you go.