In 2014, Baltimore police tracked down an attempted murder suspect with a cell-site simulator—a device that intercepts information from cell phones by mimicking the infrastructure that phones connect with. After winning a motion to suppress, the suspect turned around and sued the police, alleging a high-tech illegal search. The district court: The police had a warrant, so no dice. Fourth Circuit: But we still have to decide whether using a simulator was reasonable, and we have no idea how it works. How many other people’s phones did it search? What data can it see? Remanded for fact-finding.