Well if you read the ebay ads in the coin section you will see they area always finding these lost jars, penny banks, etc. filled with hard to find and rare coins.
Honestly the chance of rare coins and such is pretty small IMO. Most people would of either already sold their collections to a dealer, turned it in for face value, or maybe even stored it in a deposit box at bank. This is why storage units is a different form of gambling.
You do read in newspapers, yahoo online, etc. where people do find stashes of coins / cash in older homes. This is due to people not trusting banks and stuff during the depression, etc.
It's as my mother told my father and I. We can drop her off at the casino when we go do our first storage auction. That way she can't gripe about us dropping $500 on a loser if she does the same in the casino.