Are they detecting heat? Or are they detecting something else? Is there a way to move so discreetly that a motion sensor wouldn’t detect your movements?
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There’s three main types of motion-sensors:
1. Doppler radar (using microwaves)
2. Ultrasonic (technically also Doppler radar, but using high pitched sound)
3. Passive infra-red (PIR)
Microwave Doppler radar emits low-power microwave (radio) energy, an detects disturbances in the reflected energy. They are primarily sensitive to motion towards or away from the detector. They can detect motion down to a centimetre or so.
Ultrasonic emits high pitched sound, and again detects disturbances in the reflected sound. Although they are primarily sensitive to motion towards/away from the sensor, because sound bounces around a room, in practice they will often detect any motion. These will give false alarms if there’s much air motion, drafts etc, and cannot be used outside. They used to be popular intruder detectors 30+ years ago, but have gone out of favour since. Used in car alarms though.
PIR detects heat radiated by the body (think thermal camera), but they are far simpler. Owing to the way they are designed (with little segmented mirrors), they are primarily sensitive to motion across the field of view (and less sensitive to towards/away motion). If you wore thermally insulating clothes and/or draped yourself in a foil thermal blanket you may be able to evade detection from PIR, but not reliably!
Some systems use a combination of Doppler radar and PIR, to reduce false-alarms.
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