I had this issue 2 or 3 times over the last 2 years. It is horrible when it happens. Usually seems to happen in the middle of the night, and for me has been tough to trouble shoot. See the link below to my last 2 posts of the same questions, and all the responses and checks I did at the time. I don't know if I have ever really solved the problem, but here's what I came up with:
First, I replaced all the units with a modern one, still hard wired with a 9volt back up. I have more installed than I actually need in the house, but I keep them in for safety. The new ones will all flash red when they go off at the detector that triggers, so you know where the source of the alarm was. It flashes for 24 hours after the event. Good feature, because after they all go off, you would have no way of knowing the source. This feature allowed me to narrow down the source of the problems.
For me, it was one of two detectors, one in my basement, and one in the vestibule of the floor above. Suspiciously, they were both within about 10 feet of one another in the basement and first floor.
Here's where I really became lost: I had removed one possibly offending unit after it went off, and had it just laying on my counter. After 2 weeks, at 2:00 am it started going off all on its own. So I smashed it to pieces, and I thought I was done, problem solved, a bad detector. I replaced the unit where it had been, hardwired with the signal wire, and 2 months later, the BRAND NEW unit started to go off. That's when I took that unit off the signal wire, and have not had an issue since. So I think it's something with the signal wire but have no other proof. Something about that location, while other high stress areas have no such issue.
I kept the alarms up, and all units remain hard wired, but removed the signal wire from the 2 questionable locations, so if they do go off, it does not set off the whole house. I only have the signal removed for these 2 redundant units, the rest remain hard wired and with the signal wire. No issues for the last 6 months. The issue was so maddening, I stopped by the firehouse to ask their advice. The fire department thought the most likely cause was cobwebs or an ant walking through them. Possible, but I have not had the same issue since I replaced the detector but did not connect the signal wire. If it were ants, they would still be there whether I took off the signal wire off or not. So to me it is unresolved. My concern that there was some tiny short in the wiring can not be substantiated, and everyone I asked thinks it is highly unlikely, but to me I keep coming back to that question.
At any rate, should it happen again, I will check for which unit flashes red to indicate the triggered unit, and disconnect the signal wire only, and see if it continues to go off or not. If it goes off even when not connected to the signal, then the issue is with the unit itself. It takes a long time to trouble shoot random event like this, and you need to write down what happens, because if it happens again 2 months later, you'll be hard pressed to remember just what happened the previous time.
I replace the 9 volt batteries every 2 years or so, since the units are hard wire powered, they don't draw down the battery, and when the battery is low it chirps anyway. To those that say a low battery will make them all go off, the two in the vestibule that went off were both less than 6 months old with brand new batteries, while the rest of the units in the house installed at the same time or earlier had no such issues.
On the bugs and dust: I have other units, like right next to the boilers, subject to much more dust, but these have never had an issue. This is a long reply, but I am writing as much as I can think of to help you and anyone else that reads this solve their issues, because I hated this when it happened to me.
this years post
https://www.ar15.com/forums/t_1_5/1861700_.html&page=1