I saw an episode of Mythbusters on Discovery where they tried to beat a motion detector.
They said it worked by sending out high frequency waves that bounced around the entire room and then back. The sum of it all became the "fingerprint", and it should remain the same at all times.
If someone should enter the room, the "fingerprint" would obviously change and set off the alarm.
First they wanted to pad the person entering the room with thick fabric. But I mean, any body inside a room will act as a form of absorber resulting in some loss of reflecting waves to the detector. This would of course alter the "fingerprint".
It didn't work. But I'll tell you what worked:
Holding up a white bed sheet in front of you while walking through the room.
I don't get it. How could this work, theoretically speaking? I'm thinking the explanation above how the detector works must be all wrong, because physics sure works.