There was a case close-ish to me many years back- Tony Luzio, I think he has a thread on here somewhere- where the fellow was missing for something like ten years and was eventually found in a retention pond not far from where he was last seen. His car had gone into the water and either the pond wasn't checked, no one saw him, some sort of bizarre combination of circumstances and he wasn't found for close to ten years. Sometimes people just don't resurface immediately for one reason or another and wouldn't be found unless divers were available and knew where to look. (See also: Kiely Rodni.)