https://thedebrief.org/experts-warn-this-invisible-surveillance-system-is-already-built-into-wifi/
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Researchers have previously shown that WiFi signals can detect human presence or movement. BFId differs because it does not require any new equipment. Every WiFi 5 router in homes, offices, airports, and cafés already generates BFI. The infrastructure for this type of surveillance already exists, even if scientists have not yet used it for identification.."