Law enforcement's use of the surveillance devices known as stingrays, fake cell towers that can intercept communications and track phones, remains as murky as it is controversial, hidden in non-disclosure agreements and cloak-and-dagger secrecy. But a group of Seattle researchers has found a new method to track those trackers: by recruiting ridesharing vehicles as surveillance devices of their own.

For two months last year, researchers at the University of Washington paid drivers of an unidentified ridesharing service to keep custom-made sensors in the trunks of their cars, converting those vehicles into mobile cellular data collectors. They used the results to map out practically every cell tower in the cities of Seattle and Milwaukee—along with at least two anomalous transmitters they believe were likely stingrays, located at the Seattle office of the US Customs and Immigration Service, and the Seattle-Tacoma Airport.

Beyond identifying those two potential surveillance operations, the researchers say their ridesharing data-collection technique could represent a relatively cheap new way to shed more light on the use of stingrays in urban settings around the world. "We wondered, how can we scale this up to cover an entire city?" says Peter Ney, one of the University of Washington researchers who will present the study at the Privacy Enhancing Technology Symposium in July. He says they were inspired in part by the notion of "wardriving," the old hacker trick of driving around with a laptop to sniff out insecure Wi-Fi networks. "Actually, cars are a really good mechanism to distribute our sensors around and cast a wide net."