Okay, anyone reading this with access to the hotel industry should pay attention.
In my Lille hotel, the key card not only opened the door but was necessary to turn on the lights. The slot had a little nightlight to help you in the evening. You put it in the slot and left it there–the lights, if left on by the last person, then turned on. When you left the room, you took the key and had about five seconds before the lights turned out. Essentially, unless I had an extra key, I couldn’t leave my lights on. What an energy (and cost) saver. Tres bon.