A warning is being sent out to Sacramento drivers about an unmarked vehicle with lights and a siren that appeared to be law enforcement but was really a law enforcement imposter.

Writing on social media, a woman shared the incident from Highway 50 in Sacramento. She watched as an unmarked black Suburban moved from the carpool lane over quickly, ultimately tailgating her at a high speed. She reported the vehicle to the California Highway Patrol, which confirmed it was not affiliated with any department.

Over police radio from the same incident, a black car is described as having blue and red lights, moving cars out of the way in the HOV lane. The incident prompted questions from drivers: How can drivers verify an unmarked vehicle is law enforcement?

CBS Sacramento took those questions to the CHP for answers.

The CHP said it's OK to be cautious, adding that drivers should call 911, let the dispatcher know you feel unsafe and keep driving at a safe speed until you can stop somewhere well-lit and populated — or even at a police station if one is nearby.

According to the California vehicle code, officers can use unmarked vehicles when they're investigating things like car thefts, hit-and-run crashes, illegal street racing, serving warrants, or felony crimes. They're not supposed to be cruising the highways just to hand out speeding tickets.

That said, a CHP spokesperson explained in a statement that if an officer in an unmarked car sees someone driving extremely recklessly, they can make a traffic stop. The CHP says those situations are rare, though. Their undercover cars are mostly used for task force investigations and surveillance, not routine traffic enforcement.

State law requires all emergency vehicles, including patrol cars, to have at least one steady red light visible from 1,000 feet away. If an unmarked car does pull you over, it must be equipped with emergency lights and a siren. Officers will also identify themselves.