Who would have guessed?
Heat signature representation
See that thing at the bottom of the image? That is the heating unit that runs along the baseboards of my apartment for heating. In the summer/fall, my apartment needs little-to-no heating, so the design worked perfectly. But now that winter has come (the Starks are always right eventually), the unit would run constantly in the evening when the heat kicked on.
I should have anticipated this before I chose a place for it, because it makes sense what it happening. The PIR sensor is sensing the heat and notices a varying IR background. I moved the unit from the wall and mounted it on the back of my nightstand about a foot away from the heat source. All working now!