Mac Address tracking using wifi
Device Tracking: Because they're unique, MAC addresses can be used to track you. When you walk around, your smartphone scans for nearby Wi-Fi networks and broadcasts its MAC address. A company named Renew London used trash bins in the city of London to track people's movements around the city based on their MAC addresses. Apple's iOS 8 will use a random MAC address each time it scans for nearby Wi-Fi networks to prevent this sort of tracking.
The Good Apple Club