Smoke detectors frequently work by having a teensy little radioactive emitter and a teensy radioactivity detector somewhere close to it. It works a bit like a laser trap in a video game or movie, if you break the laser-beam by walking through it, it triggers the alarm because the detector can no longer see the beam.

But it doesn't have to be smoke. Water particles will do it, dust can do it, especially when there's construction work going on putting the dust in the air... it doesn't matter. It just needs to be enough to break the connection between emitter and detector.

Wet steam and the vapor from humidifiers is pretty effective at triggering it because the droplets are bigger than dry invisible steam.