The U.S. Federal Trade Commission (FTC) settled charges with Nomi Technologies over it’s opt-out policy on April 23rd. Nomi’s business is putting devices in retail stores that track MAC addresses. A MAC unique MAC address is associated with every device that can use WiFi –it is the key to communicating with your device (cell phone, tablet, laptop, etc.) as opposed to someone elses device. Nomi apparently performs a hash ‘encryption’ on this (which is still unique, just not usable for WiFi communications) and tracks your presence near or in participating retail stores world wide.
If you want to opt out of this one (of many possible) vendors of individual tracking devices you can do so at http://www.nomi.com/homepage/privacy/ .Good Luck.