Maria "Marlies" Jansen was last seen by her husband on Wednesday afternoon at her home in the Park Pacifica neighborhood, Capt. Fernando Realyvasquez said Monday.
Realyvasquez said more than 300 search-and-rescue personnel participated in ground and air searches spanning four days. Most of the searches were carried out in and around San Pedro Valley County Park.
Jansen's home is close to the park, and she was known to hike there with her husband, Realyvasquez said.
"She has a history of hiking in the park for many, many years," he said.
Jansen, who suffers from an early stage of Alzheimer's disease, is about 5 feet 5 inches tall and weighs about 100 pounds. She has brown eyes and gray-brown hair, and was last seen wearing bright yellow Crocs shoes.
The searches involved fixed-wing aircraft from the San Mateo County Sheriff's Office and the California Highway Patrol, and officers from as far away as Marin and Monterey counties, Realyvasquez said.
The search effort has been called off, though police will continue to investigate the case, he said.
"We will continue the missing-person investigation," Realyvasquez said. "We're going to follow up on any possible leads as the days go by."
Anyone with any information about the case is asked to call Pacifica police at (650) 738-7314.