I had this happen in a hotel, and being curious I removed the mirror, and yes there was a hole in the wall behind it, no there wasn't a camera there. It was just were they had ran the wiring for the lights on the mirror.

It would make sense that it serves like an access hatch to a terminal block that feeds the whole room. It's simple, costs nothing, is easy to get to (compared to having it sealed in the wall), protected from splashing and prying guests, and close to where you want most lights and outlets.