The image that you can see above is an illustration from University College London, where work is being done on a system that allows, thanks to the WiFi signal and mobile communication towers, to detect movements of people who are in another area, including protected by walls up to 25 cm wide.
The idea is to detect the Doppler effect of the movements of the person you want to detect, and use the information to determine their position at all times. Every time someone moves, they push the waves around them, and that push is what can help report as if it were a radar.
The system calculates the target positions by comparing two signals: a reference channel that uses reception of the baseline signal from the Wi-Fi access point or from another RF source, and a surveillance channel, that collects the Doppler waves generated by the moving subject.
This method has won the award Engineering Impact in the RF category and communication in the National Instruments NI Week 2015, held last week in Austin, Texas, and differs from similar alternatives in the fact that it does not have to emit anything, since it uses the waves that surround us (both WiFi and mobile towers) to do the detection work.
At the moment you can only see a point of the moving object, nothing very detailed, but it is being advanced to increase the precision, as reported in this article.