Thanks very much, I can’t believe I didn’t think of this - I think I was just so frazzled by the alarms blaring my brain couldn’t think properly. I’ll be lifting up the floor for a look tomorrow!
I presume your alarms are wired into the main electrical system. Are they also connected to each other using the red wire? This is typically used so that if one alarm detects fire/etc, all of the others start alerting as well
As an added hint, your smoke alarms might have something indicating which one specifically set them all off. Mine flashes red from that one every 30 seconds or so until it gets reset.