There are some commercially available home security systems that use a version of this tech. - https://www.kaspersky.com/blog/wifi-sensing-motion-detection...

As a homeowner, the idea of knowing the exact number of people on my property at any time is attractive. Heck, I'm pricing out a new fence to keep deer out, and it would be much cheaper to just have a system that turns the sprinklers on whenever one is detected.

But the ethical implications are wild. Unless I'm mistaken, someone could use this link today to track the activities of the couple in the apartment next door, and they would have no way to know it is happening.