Had an odd problem like this at the FD once (then later at my own house).
The 2nd floor smokes were fed by a conduit run that enetered the attic space and went above the insulation. It then re-entered the living area.
Seems the airflow was just enough to allow condensation to form and trickle into the back of the detector, causing them to sound for no reason. Removing the detector from the circuit (of course) enabled the other to function, and revealed evidence that it had been happening for a while (rust traces).
I've since heard of EC's using a dab of firecaulk or a shot of spray foam in the pipe entrance to prevent this occurrence.