The man was reported missing by his friends, who last saw him around 8:45 a.m. at the Monte Toyon center when they ate breakfast together, sheriff's deputy April Skalland said.
Monte Toyon, located at 220 Cloister Lane, is a camp and conference center used by nonprofit groups. The center is owned by a group affiliated with the Methodist Church, according to the camp's website.
When the man's friends didn't see him in a church service at the retreat, they alerted sheriff's deputies around noon, Skalland said.
"His car is still parked there," Skalland said, adding that his friends suspected the man may have gone on a hike alone.
Witnesses told deputies that around 10:30 a.m. Sunday, they saw a man matching the missing man's description in a nearby area asking for directions back to the center, Skalland said.
Skalland said the missing man has no known health problems. His name has not been released.
About 60 people from the sheriff's office and nearby law enforcement agencies are participating in the search, which involves search-and-rescue dogs, helicopters, all-terrain vehicles and mounted patrol horses.