There is probably a hole cut into the wall there.
If you want an innocent explanation, then it's entirely possible that there used to be a medicine-cabinet-style mirror there. It looks like it's about that size and would fit in that location. Those medicine cabinets are recessed into the wall and so they require a hole to be cut in the wall. When they installed the large mirror, they didn't think it was worth it to repair the hole, since they were covering it up with a mirror.
If the hotel is fairly old, it might have had something like that in the past.
But since there is no way of knowing that for sure, if I was in that room, I'd assume there's a camera behind there, too.