In general, flies and spiders can trigger single-tech/PIR-only motion sensors (this happened twice on a dedicated burglar system in the last 5-6 years, not ST, with single-tech PIR sensors). It’s supposed to be rare. Any single alarm has to be verified - it’s not worth driving home in a panic. If you got 2-3 alarms in close-time then I would be more “alarmed” but unfortunately for ST,
specifically ST motion sensors do seem to suffer false-motion-reports that seem to be only caused by cloud or infrastructure, unrelated to actual motion or PIR hardware failure. I’ve had 2-3 episodes of multiple false-motion reports on separate ST motion sensors in close-time, that were unoccupied-house with no pets and some active and passive pest controls. The spaces are monitored by cameras and no-motion was confirmed. The fact that the false-motions occurred in close-time to each other, to me strongly suggests that the false-motions originated from the cloud or infrastructure. I guessed they may be extremely lagged/late motion-reports. It hasn’t happened to me in the last 4-5 months - the first time was about 12-16 months ago.
so I don’t use ST for burglar alarm horns - just notifications to followup. I do rely a bit more on a dedicated burglar alarm system. Even dedicated burglar alarms can false, most alarms are false, so any alarm should be verified by another channel (like multiple alarm reports, camera, microphone and/or a good neighbor).