From what I have read elsewhere about this scheme, there is no electromagnetic radiation involved. The device measures the infrared radiation (heat) emitted by the body. The theory is that a concealed weapon would block some of that radiation.
FWIW, 40+ years ago I worked for a company which developed a FLIR (forward-looking infrared) device for the military. It was used in aircraft in the Viet Nam war. We used to "test" it by watching the female workers leaving the plant in the evening. The display clearly showed who was padded and whose curves were natural.
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