Your smartphone — and other devices that use Wi-Fi — broadcast a unique number when they search for nearby Wi-Fi networks. A device’s unique MAC address is sent along with “probe requests” that search for nearby Wi-Fi networks.
This tracking problem isn’t just theoretical. Advertisers in London used Wi-Fi-enabled garbage cans to track people’s movements around the city. The Wi-Fi specification wasn’t designed for a world where people carried Wi-Fi-scanning devices in their pockets all day long.