[atomwaffel]

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/

[shostack]

Really helpful, thanks for sharing.

Is disabling wifi when you are not connected sufficient to stop this sort of tracking?

[atomwaffel]

I'm not an expert, but yes, when you disable Wi-Fi, your device will stop scanning for networks and broadcasting your MAC address, so you can no longer be tracked that way. More recently, phones have also started randomising MAC addresses while scanning (starting in iOS 8, I don't know about others), so you can only be tracked for a short time as long as your Wi-Fi is on but not connected.