A Flock license plate reader linked a San Diego man to a violent crime. He was five miles away.

When Hugo Parra was arrested last year on felony charges, his pleas of innocence fell on deaf ears. San Diego police had a description of the Alfa Romeo car he was riding in and a witness who identified him during a curbside lineup as the man who brandished a handgun in Golden Hill. They had also checked the city’s automatic license plate camera system, run by the private company Flock, and got a “hit,” substantiating the claim. The problem, says attorney Alex Coolman, was that Parra was five miles away from Golden Hill at the time of the crime, and...