what invisible thing could set off my smoke detector?

(I have carbon monoxide detectors that are not going off)

I have smoke detectors that are incorporated into my home alarm system. The other day, the one by my front door went off for no apparent reason, twice, and when I changed the batteries, it started alarming again immediately.

there was absolutely no reason for it, there were no open windows or doors nearby, it just went off. so, my alarm company replaced it. installed the new smoke detector yesterday and... it just went off again. completely different smoke detector.

there's absolutely nothing in my house that could produce carbon monoxide, but I have separate CO detectors anyway that aren't going off. there's no smell, there's nothing visible, and these are those electro optical photoelectric style ones.