U.S. Customs and Border Protection has been using online ads to track people's movements and circumvent the need to obtain warrants:
https://www.404media.co/cbp-tapped-into-the-online-advertising-ecosystem-to-track-peoples-movements/
CBP Tapped Into the Online Advertising Ecosystem To Track Peoples’ Movements
An internal DHS document obtained by 404 Media shows for the first time CBP used location data sourced from the online advertising industry to track phone locations. ICE has bought access to similar tools.
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