Dante Marshall, of Alameda, was last seen at his home Sept. 27 when he left to go to a flea market at Laney College. Police said it is not clear if he attended the event and he has not been seen since.
Marshall, who sometimes appears to have the mental capacity of a 12-year-old, is without medication, which he takes daily, police said.
He is accustomed to using public transportation and has a history of going missing. In other incidents, he has been located in San Francisco, Dublin, Petaluma, Modesto, Monterey and Reno, Nev. Police said he has recently inquired about traveling to San Jose and to Disneyland in Southern California.
Marshall is described as a black man, 5 feet 10 inches, 340 pounds, with brown hair and brown eyes. He was last seen wearing a blue T-shirt, blue hooded jacket, blue jeans and white and orange shoes. Marshall may tell people his name is Michael or Bobby Hill, police said.
Law enforcement agencies throughout California and Nevada have been notified of his disappearance. Anyone with information regarding his whereabouts is asked to call Alameda police at (510) 337-8340.