US federal judge Sara Hill ruled that a sheriff's deputy in Tulsa, Oklahoma, violated a woman's Fourth Amendment rights when he searched her licence plate in the Flock Safety database without a warrant. There was no reason for the search, according to the judge - "other than the fact that [the woman's vehicle] had California licence plates".

The deputy then used her travel history from Flock as part of the justification for searching the car, where he allegedly found 91 pounds (around 41 kilograms) of methamphetamine. The judge ruled that all evidence gathered after the Flock search "must be suppressed as fruit of the poisonous tree". The Constitution didn't ask how much meth there was.

Hill didn't stop at one case. "This is a form of indiscriminate mass surveillance," she wrote. "It is not targeted at one individual. It is a tool that collects information on every vehicle passing any camera connected to the network, at all times, and serves it up to police on request." Tracking people, even in public places, becomes constitutionally problematic in her view when police can passively and indiscriminately record someone's movements over a long period and then use that data for whatever they want, whenever it suits them.

The ruling doesn't create binding precedent, but it's one of the first in which a federal judge says a Flock search is unconstitutional. It comes at a time when the system is being criticised from both left and right. Several local and state authorities, among them Florida and Texas, have announced they will stop using the technology, and Senator Bernie Sanders on Friday proposed a law, the Block Flock Act, under which federal agencies would not be allowed to use automatic licence plate readers.

Cameras that read licence plates aren't just an American story. The question the Oklahoma judge raised applies everywhere: how long is data about where we've driven kept, and who is allowed to search it without a warrant? If a system like this spreads where we live, will anyone ask that question before the cameras go up?