Your WiFi router is constantly monitoring the surrounding. You can analyze the channel state information to detect the location and even trajectory of people in their homes. There are many other applications, including detecting heartbeat, breathing rate, reading lips, etc. If you are interested in implementing one of these systems using real hardware and finding solutions to fight against it, send me an email. Note that these are rather challenging topics as they require good knowledge of communication as well as signal processing.
WiFi Sensing countermeasures
Expected gain of knowledge
Wireless communication, Signal processing