The traffic stop was in 2018. Lopez (and his lawyers) didn't find out until 2020 that it was neither the traffic offenses nor the dog that led to Lopez's downfall: It was location data from his phone, which revealed he was passing through the border at a place where there was no monitored crossing. A secret underground tunnel led from Mexico to a property he owned in the Arizona border town of San Luis.

A handful of small-town border cops hadn't been actively monitoring Lopez's phone location. They were purchasing the information from third-party brokers, who were collecting GPS data produced by the apps on Lopez's phone.

Byron Tau, then a Wall Street Journal reporter, reported that year that the federal government, particularly immigration officials, had begun purchasing such data, which had typically been meant for use by advertising companies. (It was Tau who told Lopez's lawyers about the data purchases, in the course of reporting his story.) In this way, both local and federal police were bypassing Fourth Amendment restrictions to get information that would typically require probable cause and a warrant.