People check hotel mirrors to see if there may be a camera and to see if the hotel mirror is a 2 way mirror or not.
With a camera hidden in or behind the mirror or if the hotel mirror is a 2 way mirror it can be that someone is watching the person or people in the hotel room and can also gather a persons sensitive or personal information without the knowledge or consent of the people being observed.
It's very rare that a hotel mirror would have a camera or be a 2 way mirror though.
To check a hotel mirror touch the mirror with your fingertip and if there isn't a gap between your fingertip and the reflection, it's probably a two-way mirror.
At this point, you can try to take the mirror off of the wall or keep searching for a hidden camera.
Hotel rooms are intended to be private places and the idea that someone could be watching you is unsettling.
As you may assume, there are indeed laws surrounding the use of hidden recording devices in private areas.
In the United States, it is not legal to record hotel guests in their private spaces without their consent.