Things to note: Reflective surfaces near the sensor unit can cause readings to vary drastically. Previously known causes are; mirrors and other glass, heating ducts, well polished wood, PVC piping, as well as any other shiny objects.
Reflective surfaces on the ceiling are especially problematic.
Keep in mind that these surfaces are still reflective even when the lights are off. Turning the lights off can still result in readings that are erroneous. This is due to the emitted light from the sensor unit itself, as well as other possible natural sources of infrared.
Starting closest to the unit, cover any surface that could be reflective with a dark colored (preferably black) sheet or blanket.
If covering the closest reflective surface still results in erroneous readings, please continue to cover other possible reflective surfaces further from the unit until you reach the edge of the room.
Please close doors to adjacent rooms to make sure nothing is causing light and reflections to filter into the room.