Magnetometers detect nails, screws, or the presence of metal behind the wall. Devices that use rare-earth magnets can detect metal tubes and struts, as well as nails and screws. Range-R radar devices are the closest to being able to see through walls. Range-R radar devices identify a person's location based on sound waves that bounce off a portable device.
They can see the sounds produced by the movement of a person through a wall because ultra-low frequency radio frequency waves are reflected in any object in their path, but the device only measures the distance and not what is beyond it.
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