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.

We should note that the minimal advertising shown on LWN is not delivered
via this bidding system.

https://lwn.net/Articles/1061085/
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