I have heard that as well. I always thought it was an urban myth. If it is true then it is best to return them and have them reprogrammed when hotels reuse them. I try very hard to always return them for environmental reasons. Some smaller hotels in Asia will charge you a large fee if you don't return them. As for the hotel rooms requiring a key to keep power on, I always ask for two keys even if I am the only one in the room.