Vern Erno stepped out of a car in San Luis Obispo in 2003 and was never seen again.
The 82-year-old Washington resident, who suffered from Alzheimer's disease, was visiting family in Arroyo Grande on Dec. 28 when he disappeared.
He became agitated and asked to be let out of a car he was riding in with his son and grandson. Erno was last seen at Madonna and Los Osos Valley roads.
"They drove around the block to give him some time," DePriest said. "They drove around the street, and he was gone."
Foul play was never suspected in the case, which is now cold.
"We continued to get leads for days, weeks on end, even months later," DePriest said. "All those leads were checked out. It never turned out to be him.
"We did exhaustive searches through everywhere. We were looking under trees, under bushes. We had sheriff's search and rescue. We had helicopters out. We had volunteers. It went on for days. ... It just, it's like, he vanished."