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:
How much of a drop in temperature will cause a detection? When my wife left the house it was 16 degrees, when it triggered it was 14 degrees. Surely a 2 degree drop shouldn’t be enough for that to happen? When the second trigger went off it had risen back to 16 degrees.
I have 2 motion sensors. One was positioned in the kitchen facing across the kitchen and it happened to cover the side of house window. People usually stand their and let their dogs crap on the peice of land down the side of my house. and I think it was triggering through the Glass. Moved the sensor and No more triggers.
The other one was at the top of the spiral staircase. this sensor can pick up motion in the room below at the front door. I get alot of junk mail. letter box gets left open slightly and cold draft comes in through slightly open letter box and triggers the sensor. (atleast thats what we figure is triggering it.) and again. moved position and it hasn’t miss triggered again.
Since they use IR, PIR detectors can’t detect through glass, but the window may have allowed concentrated sunlight, so it could still have been the problem.
As to how much of a temperature change, I’d be surprised if going from 16 to 14 Celsius would do it, but if it was set to be very sensitive it might. More commonly it’s around a 6° change.
It’s the change part that matters. The house warming gradually won’t cause a problem. But a gust of air significantly different then the current temperature can. You can get multiple false alarms if it’s one of those days where the house is almost warm enough to reach the setpoint on the thermostat, but not quite. So the forced air keeps coming on and off quickly. That’s why it’s often a seasonal problem.