WIFI-tracking is tracking mobile phones
WiFi tracking is mainly used by chain stores: By tracking the WiFi signal of a mobile phone, they know how many people walk along the shop, and how many of those people actually go in. One identifies the user on the basis of the unique MAC address, and so can be seen whether a person (or actually a specific MAC address) comes back regularly or not. Tracking also often takes place with general GSM signals and Bluetooth but WiFi is more accurate and Bluetooth is often on phones. Beginning 2013 came to light that things like Dixons, Mycom and iCentre have this kind of system in use, in the United States this has been going on for a while, and there have also been questions about privacy (while privacy is much less important than in the European world).