i just found this site today and I'm sure you have solved your problem, if not try this. Some motion dectors have an adjustment (pet) screw on the bottom where you can turn it towards the neg direction. This moves a inner wall insde the sensor upward to block the bottom portion of the sensor off. I don't think xfinity has this feature yet, but you can take a piece of tape to cover a portion of the sensor on the bottom.
... View more