Close-In Surveillance Using Your Phone's Wi-Fi
This article talks about applications in retail, but the possibilities are endless.
Every smartphone these days comes equipped with a WiFi card. When the card is on and looking for networks to join, it’s detectable by local routers. In your home, the router connects to your device, and then voila you have the Internet on your phone. But in a retail environment, other in-store equipment can pick up your WiFi card, learn your device’s unique ID number and use it to keep tabs on that device over time as you move through the store.
This gives offline companies the power to get incredibly specific data about how their customers behave. You could say it’s the physical version of what Web-based vendors have spent millions of dollars trying to perfect the science of behavioral tracking.
Basically, the system is using the MAC address to identify individual devices. Another article on the system is here.