I'll defer to experts, but it appears to be an Olive Ridley Sea Turtle (Lepidochelys olivacea). [Note the gray coloration, and that there are at least 6 costal scutes (or scales) — these are the long scutes located between the vertebral scutes (running down the middle of the shell) and the marginal scutes (along the edge).
Perhaps you heard that an Olive Ridley was spotted off Point Reyes in late December? Folks have been wondering if sea turtle sightings in northern California this year could be related to El Niño conditions.
If you encounter a sea turtle on a local beach, you can call The Marine Mammal Center or the California Academy of Sciences.
P.S. Click here for a picture of the only other sea turtle I've seen on Salmon Creek Beach (on 18 September 2013).