Retail stores use bluetooth beacons to track customers extensively

The grocery store got your location data and paid a shadowy group of marketers to use that information to target you with ads. But the location data industry has a much more precise, and intrusive, tool: Bluetooth beacons. Let’s say you visit Target; it might be collecting data from you, but it might rent its beacons out to other businesses, allowing them to monitor your location.

Source: www.nytimes.com