Already exists in prod. At a meetup several years ago, it was demonstrated that you could track a person’s location in an adjacent room by triangulating Wi-Fi interference.

I’m not into RF so this is a bit imprecise, but it took like a RPi with GNU radio, a second router, and some code to diff the signals into a location.

The demos were compelling, you could see a heatmap of a user waving and so on. This is a product already sold.