"If you suspect the person in the unmarked car is not a police officer," Sergeant Sherman Hall said, "calling 9-1-1 is a reasonable way to verify the authenticity."
The 9-1-1 dispatcher can tell if the stop is legitimate; if not they can send help. "If you don't have a telephone, and have genuine concerns, head for a well-lit, populated location," he said.
He added that the law requiring hands-free use of cell phones while driving has an exception for calling 9-1-1. A genuine police officer will show a badge and police department photo identification, he said.
The incident occurred on Wednesday, Nov. 30, when a 23-year-old woman driving a 2007 BMW pulled over at Atherton Avenue at Elena Avenue when she saw a car with a blue light on its roof following her.
"The suspect approached her and asked for her driver's license and registration, as well as her car keys," Sgt. Hall said.
When the woman asked why she had been stopped, she was told she was following the vehicle in front of her too closely.
"The suspect asked her if she had had anything to drink," Sgt. Hall said. "He also asked if there was anything edible in the car." The man gave the woman back her driver's license and registration but instead of returning her keys, drove away.
Sgt. Hall said the woman initially believed the "officer" might be repositioning his car so she waited a short time for him to return before she called a family member to bring a spare set of car keys so she could drive home.
"Upon reflecting upon the unusual nature of the encounter, she called us about 20 minutes after the event." Sgt. Hall said. "We searched the area, but did not find the suspect."
Because the fake officer had her keys and address, the Atherton police did "keep special watch over the car," he said.
The man was described as a Hispanic or Filipino male adult between 27- and 50-years-old driving a dark Honda or Toyota. He was not in a police uniform.
"These kinds of incidents are unusual," Sgt. Hall said. "We take them very seriously. In fact, we immediately alerted neighboring agencies."
Only about a handful of such incidents occur in Northern California each year, he said, and senior officers in Atherton could not recall this ever happening in Atherton before.
Sgt. Hall also said in California every emergency vehicle must display a forward facing, solid red light. Blue lights are OK, but the cluster of lights must include a solid red one, he said.