It does not mean the room is clean. Everything on this page finds devices that give themselves away: a lens that retroreflects, a magnet holding a mount, a client on a router you can log into, a cable going somewhere odd.
A professional sweep uses an RF spectrum analyser, which sees transmitters across the band including ones idle most of the time, and a non-linear junction detector, which finds semiconductor junctions in a wall or a chair leg whether the device is powered, switched off, or has no battery at all. Neither has a browser equivalent, and no phone app substitutes for them.
If the stakes are high, hiring someone with that equipment is the actual answer.
A camera is placed to see something. Whoever put it there stood where you are standing and worked out where it had to go to cover the bed, the desk or the door. The set of places that satisfy that is small, and the set of places a camera could physically sit is large.
Searching object by object walks the large set. Searching sightline by sightline walks the small one, which is why it finds things faster.
Press a fingernail to the glass. On an ordinary mirror the silvering sits behind the glass, so your nail and its reflection are separated by the thickness of the pane and you see a gap. On a two-way mirror the reflective coating is on the front surface, so nail and reflection appear to touch.
Thin front-surface mirrors exist and thick decorative glass exists, so the gap varies for reasons that have nothing to do with surveillance. Treat the result as suggestive.
The reliable check is light. Turn every light in the room off, cup your hands against the glass, and shine a torch through it. A mirror with a room behind it shows you that room. A mirror against a wall shows you nothing.
If you can log into the router, the client list is the strongest evidence on this page: a device that is on the network is named, addressed, and has an OUI you can resolve to a manufacturer.
It shows only devices on that network. A camera on its own hidden SSID, on a neighbour's Wi-Fi, on a 4G SIM, or recording to a local SD card with no network at all appears nowhere in that list. An empty-looking client list rules out one class of device.
Photograph it where it is, before touching it, with something in frame that establishes the location. Note the time. Keep the original file rather than a screenshot of it.
Moving or dismantling a device destroys the placement evidence and may destroy fingerprints and any recording still on it. In a rental, a workplace, a changing room or a bathroom, this is a criminal matter in most jurisdictions, and preserving the scene matters more than removing the device.
If you are not safe in the room, leaving is the first step and the device stays where it is.