Wi-Fi and Bluetooth devices around you: mass surveillance.

Geolocation is not only done by triangulation of mobile antennas. It is also done using the Wi-Fi and Bluetooth devices around you.

Operating system manufacturers such as Google (Android) and Apple (IOS) maintain a handy database of most Wi-Fi access points and Bluetooth devices, as well as their location. When your Android smartphone or iPhone is turned on (and not in Flat mode), it will actively scan (unless you specifically disable this feature in the settings) for Wi-Fi access points and Bluetooth devices around you and will be able to geolocate you more accurately than when using GPS. This active and continuous polling can be sent to Google/Apple/Microsoft as part of your telemetry.

The problem is that this polling is unique and can be used to uniquely identify a user and track them. Stores, for example, can use this technique to fingerprint customers, including when they return, where they enter the store and how long they stay in a particular location.

This allows them to provide accurate locations even when GPS is turned off, but also allows them to keep a convenient record of all Bluetooth Wi-Fi devices around the world. Which can then be accessed by them or third parties for tracking.