These motion sensors are actually measuring change in temperature that moves across the sensor.
It’s quite common to get false alerts as the weather starts to change and the heating system in the house comes on at different times for different durations.
Also, if direct sunlight falls on the sensor and then a cloud moves across the sun, it can be treated as a motion alert.
Finally a spider or other insect actually crawling across the face of the sensor can trigger it. They won’t trigger it from further away but it does happen.
All these reasons are why many people rely on zone detection for security systems, rather than just one motion sensor. If you are already using a zone then it could still be the HVAC, but it’s probably not the other two.
If you’re not using a zone, you might want to consider using one. @Mike_Maxwell has a good smart app for setting up zone detection:
Smart Zone motion detector (Zone Motion Manager) 2.1.0 release