The driver left the 2000 BMW 528 running and the doors unlocked while he ran into the store at 1901 Bridgeway around 12:50 a.m., police Lt. Kurtis Skoog said.
The suspect got into the car and fled east on Easterby Street, Skoog said.
Officers from the Sausalito and Mill Valley police departments and the Marin County Sheriff's Office searched for the stolen car but did not find it, Skoog said.
It was the fourth auto theft since Oct. 12, police said. All the vehicles were unlocked with the keys inside.
A 2003 BMW X5 was stolen from a driveway on Spencer Court on Oct. 15, a 2002 Ford Escape was stolen from the 50 block of Filbert Street on Oct. 14, and a 2000 Acura TL was taken from the 400 block of Sausalito Boulevard on Oct. 12, according to police.
Police have found two of the vehicles and are reminding residents to lock their vehicles and remove any spare keys inside them, Skoog said.