Gregory's Server
Some great research from Germany. The journalists were able to get a “preview” from a data broker with locations of 11 million German advertising IDs over the period of two months. For free, no questions asked, merely claiming to be interested in buying a subscription.
The dataset appears to be compiled from multiple sources and has some quality issues: some locations are only approximate, occasional wrong timestamps, duplicate entries with different advertising IDs. Yet in many cases it is easily possible to find the person behind the movement profile and to learn details about their lives that definitely weren’t meant to be public knowledge.
That’s your installed apps (or rather advertising SDKs they are built with) selling whatever data they can get to anyone willing to pay. I wish I could recommend disabling GPS and the issue is solved. But even though GPS is the source of the most precise location data, it isn’t the only one. The data broker industry is out of control.