Police in one Bay Area city expected to spend Friday night looking for impaired drivers. Instead, they found themselves pulling over a car with no driver at all.

During a grant-funded DUI enforcement operation in San Bruno, officers watched as one of Waymo’s autonomous vehicles made an illegal U-turn at a traffic light. They stopped the car, only to discover an empty driver’s seat.

“No driver, no hands, no clue,” the department wrote in a social media post.