Source: Brian Fung - The Washington Post

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.

Read Full Article: Stores use your phone’s WiFi to track your shopping habits