https://techcrunch.com/2018/09/25/new-tech-uses-wi-fi-to-count-people-through-walls/

Whether you’re trying to figure out how many students are attending your lectures or how many evil aliens have taken your Space Force brethren hostage, Wi-Fi can now be used to count them all.

The system, created by researchers at UC Santa Barbara, uses a single Wi-Fi router outside of the room to measure attenuation and signal drops. From the release: