I had a case about a year ago where a woman was renovating her house and she found a bunch of cards in the wall. Posted about it on the NonSport Update forum. I would up buying seven R152 Tootsie Circus cards from her which graded out between PSA3 to PSA4.
A friend of mine bought all the 1936 Fleer Cops & Robbers cards she had.
So it does happen. My theory is that a child hid them, in the wall from his/her siblings and forgot about them.