Recently my alarm kept triggering and I couldn't figure out why. I first figured out which motion sensor was causing it. That helped a bit, but not enough. I then boosted up the logging and discovered (1) it happened very rarely when nobody was home and (2) also around the time the heater went on or off. At first it seemed to happen mostly when the heater went off so I thought the sensor was too close to the vent. I didn't want to move the sensor so I programmed around it. Then I discovered it also happened while the heater was on (not quite as often), so I programmed around this as well. Finally I couldn't get rid of ALL of the false positives so I took the top off the sensor (which is mounted to the ceiling) and had the cap in my hand while looking at the sensor on the ceiling. Everything looked fine, the senor was already on least-sensitivity.
I then looked down in my hand and there was a silverfish sitting on the PIR lens. Once the bug was removed from the system, the false positives were 100% gone!