I grew up incorrectly believing that motion detectors (as part of burglar alarm systems) were designed to detect movement, when in actual fact it is the movement of a heated (like body heat) object which causes them to trip. I also understand there are some which are so sensitive they can even detect the body heat from fingers typing on a keyboard!
Question #1: Are there motion detectors which just detect motion, without the moving object having to radiate heat?
I ask this question because the above sensors can be blinded by a small glass picture frame placed over them, with a long pole. It would be much more difficult to bypass if they detected movement.
I assume we should exclude sonar sensors since they are easily bypassed with a thin layer of fabric, such as wearing a bed sheet like a ghost, according to Myth Busters.
Question #2: Aside from microwave motion detectors potentially being problematic when they can detect movement through solid objects such as wooden walls, furniture, and glass which could lead to false alarms - why are they not used more frequently?
Question #3: Why do some PIRs in alarm systems have three LED lights yellow | red | green under the plastic sensor? I ask as perhaps they detect three different types of movement, so this might be related to the question and not off-topic.