Robert Davis, 93, was last seen at 8 p.m. Wednesday after leaving Marin Joe's Restaurant in Corte Madera and no one's heard from him since.
Ryan Mathews has spent hours online uploading pictures and sharing posts in order to find his grandfather.
"I'm worried. It's really difficult to try to visualize where he could be," Mathews said.
Davis left Corte Madera following his weekly dinner with his daughter and was last seen driving away from the restaurant toward U.S. Highway 101. He was supposed to drive straight home to San Rafael, but he never made it.
"It was a rainy night and he is 93, and he does get somewhat confused sometimes, has short term memory loss," San Rafael Police Department spokesperson Margo Rohrbacher said.
"We have no idea what he could have done. We just don't know why we can't find him," daughter Leslie Mathews said.
An air search Thursday and a ground search Friday revealed nothing.
Davis' family has driven 400 miles searching back roads, ravines, alleys, anywhere they can think of, but still no sign of the World War II veteran.
Davis was driving a 2001 silver Toyota Camry with a California license plate 4SEY056. The car is missing a right front hubcap and may have damage to one of the fenders.
Police said Davis suffers from a medical condition and may have become disoriented. It was raining heavily when he left so he may have gotten lost on his way home.
Davis is a white man with gray hair and gray eyes who stands 5 feet 7 inches tall and weighs 139 pounds. He was wearing a beige jacket, yellow sweater and beige slacks when he was last seen
"You can't drive forever, there has to be an explanation to where he could be. You don't just vanish," Ryan said.
Davis has no cellphone and has not used his credit card.
Not calling his family after this long is something he simply wouldn't do.
"He's a creature of habit and always on time and he's happy and he always said, 'I'm going to live until I'm 100,'" Leslie said.
Anyone who has seen him is asked to call San Rafael police at (415) 485-3000 or 911.
Bay City News contributed to this story.