What is this thing on the ceiling of the hotel? There is a small red light flashing. Not the camera inside, right?

It's called a cigarette lighter. It's used to report fire. When the smoke or dust in the room reaches a certain concentration, the detector will send back a fire alarm signal to the fire control room. The alarm is that the signal light is always on and flashes every 3 seconds.

Note that the detector has a coded address, so the duty room can clearly locate which room the alarm is in after the alarm.