A vintage home security job has me puzzled by false triggers

I set up a full alarm system in a house from the 1920s a few days ago. Now the motion detectors go off randomly, often at night when the place is empty. I rechecked all the sensors and wiring, and it all looks correct. Even tried adjusting the sensitivity, but no luck. Maybe it's the old floorboards settling or drafts from original windows. Has anyone else run into this with older buildings? What did you do to make it stop?