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.