Point Reyes National Seashore rangers found Katherine Truitt's vehicle Friday afternoon. When they saw the Ford again Saturday they called the sheriff's office and asked for help.
About 40 people from the National Park Service, the U.S. Coast Guard, the California Rescue Dog Association and various search and rescue teams from around the Bay Area looked for her Saturday, Marin county sheriff's Sgt. Debra Barry said.
Some 80 people returned to the area at sunrise Sunday to continue looking. They searched until dark.
Truitt's niece, Sarah Montoya, said she is flying up from Orange County on Monday to help look for her aunt. Montoya's grandparents were with the search teams in the area of McClures Beach Sunday.
The woman's family said she usually goes out there and hikes. They are very worried.
Bay City News contributed to this report.