...they never trigger on anything but a living thing moving.
They will also get triggered by vehicles passing within the targeted area. This may be due to the difference in heat as you said. I have several motion sensor lights on the outside of my house, and I have them set to go off if anyone drives into my driveway. Both my garage flood light and my two lantern type light fixtures on my front porch will go off anytime a car comes or goes in or out of my driveway when it is dark.
So anything but a living thing setting them off is not 100 percent correct.
I just looked at every motion sensor light users manual that I have, and they all say that any change in temperature can set them off. They recommend that as you set up the fixtures, that you maintain a good distance between the actual motion sensor and the light bulbs as possible in order to lower the chances of false triggering of the motion sensor due to heat differentials.
It also recommends that you don't point the motion sensor at any "metal" objects, as the temperature change in the metal versus the surrounding area can be different, and that can trigger the motion sensor as well.
Understand that it will light up with wind, insects, snow, rain, etc.etc.
Jim
The motion sensor light on my shed will trigger if it gets too windy, and the maple tree next to the shed that has leaves on it starts to sway in the wind. It has to be really windy though. It won't trigger in a slight breeze.
Jim