Police said Saturday the man fled after the driver told him she was calling the California Highway Patrol.
The bus driver says she stopped Friday afternoon when a silver-colored sedan pulled behind her and began flashing blue-and-red lights.
A man in his 30s, who was dressed in civilian clothes, emerged from the car and told her he was a police officer.
He flashed a badge and had a card clipped to his shirt with the word "Investigator" written across the top in red.
The driver says the man got back into his car and drove away after she told him she was calling the CHP.