Once you start playing with radiowaves and antenna you start noticing the intricate ways it plays with and around bags of water like bodies. I'm sure the original research on location/movement tracking was due to scientists trying not to get interference, later once they figured it out it was natural to see how much data they could get out of a radio interference profile.

I remember the original tech was going to be marketed as a way to tell if your old person (parent etc) had fallen down and stopped moving. Not the best use case, and then the privacy implications became clear. Once that happens the race begins to exploit the tech.

...But the eventuality here is something like a Star Trek tricorder that can take multiple vitals and detect irregularities from across the waiting room. Sensors that remember who was in a room and what settings they had. Etc. Some cool thing besides the bad stuff (microtarget those ads).