what invisible thing could set off my smoke detector?

I found it amusing that these posts were adjacent.

Call the fire department, they have detectors that they can use to look for gas leaks and other things that can set off a detector.

You can also call your gas provider. One of those two should be able to track it down, it could be a lot of things, but two different smoke detectors going off in the same location is a huge red flag.

Best case, you have something kicking up fine dust, worst case, you have a smouldering electric fire in your wall somewhere.

Don't panic, but also do not ignore this.

Look for an expiration date. Radionucleotide style detectors end up failing with false positives when they reach end of life. You might need to have all the old ones replaced.

I don't think this is what you're experiencing, but I had an alarm go off randomly for one beep once. Went and looked at it, and a few seconds later a spider crawled out and away from it.

If it's photoelectric, anything that could scatter light could cause it to go off. Is your house dusty?

Mine went off in high humidity when I showered and forgot to turn on the fan

I’d call the fire department to ask them to come out and make sure that there’s not anything slow burning that’s hidden in the walls. Be sure to mention two separate smoke detectors have been going off. Even if that’s not what it is they’ll be fine with coming out to check.

High humidity can cause them to go off as well. Used to use a cool air humidifier in our kids' room at night and had to stop because it would eventually set the alarm off.

I've had a photoelectric alarm set off by steam from a dryer in the next room, through a closed door.

A ghost 👻

Spiderwebs or insects can mess with the sensors, likewise with dust. Try spraying some canned air inside. Or if it's a few years old, you may want to replace it.

I've set one off while dying my hair.

I think you said elsewhere that they are new and that they are photoelectric rather than the radioactive ones.

I'm purely taking a shot in the dark but I'm wondering if you should try sealing up the hole(s) in the wall that you made to run electric and to mount the detector

My thinking is that dust might be getting caught up in a tiny draft through that hole and it's so close to the source that it sets it off. Cause like, if wind hits the side of the house, there can be some positive pressure in the crawlspace which often also means inside the walls.

I guess maybe somehow there could be some stream or condensation as well. If it's right by the front door and the humidity is high, maybe the hot air from outside meets the AC air and causes a tiny amount of condensation. Or if you live in hellscape temperatures, maybe there could be some vapor generated because of the hot air.

Dust maybe? Dust can have the same particles as smoke.

I have set off smoke detectors with a power washer and with saw dust

I need to replace a set of expired smoke detector at my elderly parents' house. They're too old to have alarms going off in the middle of the night just because the wind blows.

Yet, Amazon only seem to sell ones with photo-electric sensors, and many reviews complain about over-sensitivity with dust, and under-sensitivity when the room is clearly full of smoke.

Additionally, the ones with sealed 10 year batteries - many reviews report a battery life of 2 years or so in practice, with increased false positives as battery life runs down. So now, they have to replace whole units rather than just batteries.

What happened to good old ionizing smoke detectors with 9v batteries that needed replacing every 2 years or so?!

Might be worth buying an infrared thermometer to see if there is wiring overheating in your walls. I'm not an electrician but i wonder if it's something then can happen sporadically such as if there is something drawing a lot of power that turns on/off. There are inexpensive ones out there and they can be handy to have around.

Are they Kidde? I've had 3 photoelectric Kiddes that started failing and going off randomly. I've been slowly replacing all of mine.

The builder installed them. Occasionally walking through our neighbourhood we hear other people's going off too.

They are probably both about the same age and need replacing. Smoke and carbon monoxide detectors have a finite lifespan, no matter how often you change the batteries. Fortunately they're not all that expensive, just get new ones. I had the same problem in my apartment last year, and the carbon monoxide detector was over 10 years old. So they just replaced it, problem solved.

Does it get hot by your front door or in direct sunlight? We have a few in my house that go off if our kitchen gets too hot. We had to move them down the hall slightly and they stopped. A really old one we have upstairs, hardwired into the house electric (built in 86) trips if too much steam builds up in the bathroom and let's loose into the hall.

Since it went off in your hands, have you tried googling the make and model to see if there are any similar complaints or even recalls for them?

Does anyone in your house vape?

I had trouble with them going off in humidity, they were past their expiry date so replacing them fixed the issue.

Are they networked? Mine are somehow connected and the one that beeps doesn't always seem to be the one that detected the issue.

Some newer (in the last 10 years) smoke detectors use an infrared sensor to detect fire as well as smoke, and it may be going haywire off a reflection of the sun, or intense heating of a spot within its detection area.

If you can, borrow a FLIR or infrared camera and check the area when the detector goes off.

If you post your model of smoke detector, it would be easier to tell if it has this feature.

Did you open one up yet? That might give you a clue 🤔

E.g. I don't know exactly how wide the gaps are, but here it looks like small insects could get in. Maybe you have another problem than smoke 🫣

What country? AFAIK in the US you can't make the batteries replaceable. If they are wirelessly linked they can have auxiliary batteries for that, but (I believe) that's different than the main battery...

EDIT: I seem to be thinking of California, maybe not all of US.

Depends on what kind of detector it is but alot of them use small amounts of radiation and a detector that triggers when the number of particles detected drops below some level.

How smoke detectors work

That being the case any particulate large enough to interrupt the particles could cause it to go off.

For example high humidity misty water from a shower wafting over a detector placed over the bathroom door, etc.

Have you ruled out other types of fumes? Eg fresh paint, perfume, scented oil diffusers etc.

Are your smoke detectors linked to each other? Could be faulty wiring in the circuit, or a completely different smoke detector failing and sending out an alarm that triggers the others. The latter happened in my home when I was growing up: the living room smoke detector kept going off a few seconds before the rest of them would chime in, but it turned out it was the one in the nearest hallway that was failing and sending out bad signals. The living room detector was just the next in the circuit.

Try replacing the batteries. That's often the reason for this type of thing.