License plate readers installed on highways, bridges and tow trucks have been used for years by federal agents, state traffic authorities and local police. They’ve played a role in rescuing kidnapped babies, tracking down U.S. Capitol riot suspects and pursuing undocumented immigrants. But Flock’s founding, in 2017, helped adapt vehicle surveillance for the general public, offering residential installations, consumer-friendly interfaces and advertising tailored to a suburban clientele. (Flock’s main competitor, Vigilant Solutions, sells primarily to local governments and police.)