The Earth's field runs roughly 25 to 65 µT depending on latitude, and your phone adds its own offset from the speaker magnet and the case. So an absolute reading of 48 µT tells you nothing on its own.
Set the baseline in the middle of the room, arm out, away from walls, radiators, laptops and door frames. Everything after that is measured against it.
The sensor is a magnetometer on the phone's mainboard, so the reading is strongest with the back of the phone a few centimetres from the surface you are checking.
A dipole field falls off as 1/r³. A small neodymium mount magnet reads in the hundreds of µT when the phone is touching it and drops into single digits by 20 cm, which is why the sweep only works held close and moved slowly.
Hand shake, sensor noise and turning the phone in the Earth's field move the number by a few µT on their own. Under about 5 µT you are looking at noise.
15 µT is the default: comfortably above that noise floor, and reached by a typical mount magnet at a few centimetres. Drop to 8 µT for a slow, careful sweep of a small object. Raise it to 30 µT in a building where the structure itself is loud.
Steel studs behind plasterboard, rebar in concrete, mains wiring under load, radiators, metal door frames and hinges, HVAC ducting, screws, nails, curtain rails, hard drives, laptop and tablet magnets, speakers, headphones, magnetic phone mounts, fridge doors.
A steel stud gives a broad rise that stays up as you slide along it. A mount magnet gives a narrow spike that peaks and falls off within a few centimetres. Sweep past a hit twice from different directions before you take it seriously.
A camera in a plastic housing with no magnet in it, screwed or taped or glued in place, will not show up at all. Neither will one far enough inside a wall for the field to have fallen away.
A reading under the threshold means the phone measured no magnetic anomaly on the path it was moved along. It does not mean the room is clean, and it does not mean the surface is clean if you swept it quickly or from more than a few centimetres.
Run this alongside the lens finder, which looks for the optic instead of the mount, and the room sweep checklist.
Every reading stays in this tab. Nothing is uploaded, and the CSV export is written by your own browser.
The Generic Sensor API needs Android Chrome or another Chromium browser on Android, over HTTPS. iOS Safari exposes no magnetometer to a web page, so on an iPhone this test cannot run at all.