Following a tip-off from robotaxi firm Waymo, San Francisco police conducted a “high-risk vehicle stop” and arrested two juveniles last week. The crime? The Waymo had seen its two passengers handling a loaded AR-style assault rifle. Police who intercepted the car also found “suspected marijuana and mace spray,” reports SFGate.

Fully autonomous taxi cabs like the Waymo in this story are an increasingly common form of transport seen on the world’s roads. With no human driver, these electric vehicles are packed with sensors, cameras, and more, all tied into the software presence that is called the Waymo Driver. To ensure the utmost safety, the sensors also continually monitor passengers. This might typically check if passengers are following the rules regarding minor transgressions like smoking or not wearing a seatbelt. In this case, the cameras recognized something more serious.