I have a back bedroom with sealed windows, and it gets dry sometimes. It’s for a person with horrible allergies. This room is so clean you could eat off the floor so to speak. I run a humidifier set to 60% with distilled water for the person in there. When I checked on it last night to see if it needed to be filled, the humidity had gone up to 82%. This was because a ceiling fan had been left on and the sensor in the humidifier wasn’t getting correct data from the moving air (I think). My hallway smoke / Co2 detector went off as soon as I opened the bedroom door. It would not clear until I pointed a hair dryer at it, then it shut the hell up.
TLDR: My smoke detector doesn’t like humidity all of a sudden?? and went off and would not clear. It has been more humid than that in the entire house without humidifiers running, and I have the same detectors in other locations with no issues.
I had an old one that would go off every time anyone would take a shower. Just another vote for steam setting it off.
Certain types of smoke detectors dont detect ‘smoke’ or ‘steam’ exactly, but they monitor a small section of air in the unit and if there is enough particulate matter in the air the sensor is triggered and it goes off.
Because of this they are sensitive to aresols too, such as spray on deoderant.
This is almost certainly the correct answer. If it’s a photoelectric-type detector it’ll be set off by any interruption of the beam. This can be smoke, steam/vapor, dust, etc.
You can replace them with ionization type smoke alarms (or “dual type” alarms) and this will make it less sensitive to non-smoke interference.
This is also why vaping can set off smoke alarms.