Oh gosh, I wish this old thread would die! lol

Using a camera would need a bunch of image processing, I guess, to detect the person moving past. Even easier might be to leave the phone on the sofa and use the accelerometer code from maeHammer (part 2 of the tutorial here - 'rt2 - adding sound and accelerometer with maeHammer') to play back the noise when the phone is moved/jolted when the victim sits down. Just make sure you don't hide the phone too well or the person may sit down on top of it