A team of researchers from Carnegie Mellon University developed an algorithm that helped detect human presence through walls using WiFi signals from a $30 router.

“Using routers to ‘sense’ humans is proposed as an alternative to RGB cameras (that have sensors catching colored images of persons and objects). The researchers suggest that the advantage is the fact routers are almost omnipresent in closed spaces, while the new method solves the lighting, etc., problems when using cameras.”