“A tiny camera or some sort of detector on the ceiling? Found in a waxing salon.”
Answer: That light and test button are generally required on the emergency lights required in commercial buildings by fire and building codes.
New lighting fixtures use so little power that it makes sense just the build a backup battery into the fixture. It saves having to wire and mount a conventional “emergency light.”
Fun fact: most commercial buildings have “night lights” – a few lighting fixtures scattered around the building that have no switch and are always on. Battery-backup fixtures are also usually “night lights”.