Retail stores use Bluetooth beacons to track customers

But the location data industry has a much more precise, and unobtrusive, 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. But there is nothing theoretical about Bluetooth beacon technology that follows you into retail stores (and other venues) and tracks your movement down to the meter.

Source: www.nytimes.com