Via a Linux Weekly News article:

CBP Tapped Into the Online Advertising Ecosystem To Track Peoples’ Movements (404 Media)

This 404 Media article looks at how the US Customs and Border Protection agency (CBP) is using location data from phones to track the location of people of interest.

    Specifically, CBP says the data was in part sourced via real-time bidding, or RTB. Whenever an advertisement is displayed inside an app, a near instantaneous bidding process happens with companies vying to have their advert served to a certain demographic. A side effect of this is that surveillance firms, or rogue advertising companies working on their behalf, can observe this process and siphon information about mobile phones, including their location. All of this is essentially invisible to an ordinary phone user, but happens constantly.

I have no idea whether this applies also to web browsers with ad-blocking extensions loaded (I use uBlock Origin and Privacy Badger in Firefox). Apps are basically web browsers in handcuffs that can't have ad-blockers attached, so the user's in handcuffs as well.

Protest safely out there.