Retailers can track customers through their mobile devices.

Every smartphones has a unique 12-character MAC address that's visible when it's searching for WiFi or Bluetooth. No personal information is divulged, but it can tell retailers where that phone goes in the store and whether it's visited before.

When tracked, the number can help retailers determine whether sales or window displays draw in more customers, the wait time in check-out lines and how long a shopper stands in a certain department.

Now, if you join a retailer's Wi-Fi network, anonymity goes out the window. Accepting the terms allows a retailer to directly market to a shopper.

Don't want to be tracked? Disable WiFi.

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