Judy Gilbert was last seen on Vista Linda Drive in Mill Valley near the Camino Alto Open Space Preserve.
Search and rescue crews have been combing the area looking for any sign of her.
Investigators say Gilbert frequently walks there, but she left her phone at home, so they aren't able to track her that way.
They believe she took her purse with her, but they don't know if she has food or medication. They believe she's alone.
"Our search and rescue teams have some canine teams that have been utilized in the field and we're hopeful that some of them would be able to pick up a scent, and it hasn't been fruitful, yet, but we're hoping that, that will continue," said Sgt. David Kohler with the Mill Valley Police Department.
She is described as 5 foot 4 inches tall with blonde hair and hazel eyes, and weighs 90 pounds.
Authorities say she could have been wearing a navy blue jacket, black pants and a purple/gray beanie.
Investigators are asking anyone who lives near the Camino Alto Open Space Preserve to check their home cameras to see if Gilbert may have been spotted.
If anyone sees her they are asked to call 911.
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