Not the original poster, but when you sign into a Wi-Fi network, the router needs to know where to send the packets, so it needs to have a way to identify your device. That's why all devices that can connect to a network have what's called a MAC address[1] – a very long number that is unique to your device. Your device also broadcasts that address when it scans for networks, so just about all the time. When you control several access points and a few of them see the same device at the same time, you can triangulate the location of that device down to a few metres.

The company I used to work for did something similar[2] at a conference over the course of three days. (You can drag on the map to highlight individual devices.) It's really cool and really quite creepy.

[1]: https://en.wikipedia.org/wiki/MAC_address

[2]: https://apps.opendatacity.de/relog/