The only story I've heard from someone on another very reputable coin forum was that they bought some wheat cents in a baggie that some guy had found hidden in the back of his closet. The seller didn't claim that they were 'unsearched,' just that he had found 'em and didn't want 'em. The buyer (who was/is well established on this other forum) found an AU58 1955 double die in the baggie. Coin was authenticated and that was that.
I'm sure there have been legit finds, but with the advent of 'salting' rolls, who knows anymore.