Many hotel rooms in Europe, Mexico and beyond require the room key to be placed into a receptacle to power the room on. This is brilliant for energy savings, but it takes some getting used to.
In Germany, I had to make two different trips to the desk to request a new key as I walked out without mine. Shame on me…but I often leave my TV on when I am leaving the room so that habit overrode the one to check for my key in my purse before I walked out. :/. Thankfully this receptacle is most always right by the front door so it should be a good reminder before you head out.
As a hint, if you need to use your key (maybe someone else in your room needs to go somewhere and needs a key later), often – but not always – you can also slide the envelope for your key in this slot as a placeholder.