Scientists in Germany have demonstrated a startling new form of surveillance: identifying people using nothing more than ordinary WiFi signals. By analyzing how radio waves bounce around a room, researchers can effectively “see” and recognize individuals — even if they are not carrying a device and even if their phone is turned off.
I don’t think a person has to carry anything. The tracking is based on measuring the interference a moving person (or a dog) creates between the router and a connected device like a range extender or a networked printer.