If you're worried about a moving item like a fan causing false alarms on motion detection systems, look into ZoneMinder. It lets you use a webcam to monitor a room and checks the checksum value of the image to see if the image has changed, thus detecting motion. But the neat thing about Zoneminder is it allows you to draw areas on the image that you want ignored. So you can set it up to ignore your cats walking around down low on the floor, or put a camera outside and draw an area around the clothes line. Check out the ZoneMinder site.