I don't know why they do this as I don't believe it is a law here in WA that they must.
I've never seen a law, either.
However, every auctioneer around here has said if they see them, they take them.....so if not in open, don't tell them!
Once taken, King County Police will run to see if stolen, you bring receipt from auction and pass background check and they "may" return them. Local police it's very unlikely you'll see them again.
They can claim only the last registered owner can pick it up which they don't have the resources to even know who the last registered owner was unless its only been one owner or buyer sent in transfer paperwork. Or they claim the locker owner owns it because they took before you bought the unit.
Both are BS from a legal standpoint but to fight it would be more money/time than the firearm is worth in most cases.
My opinion is once LE gets their hands on valuables, it's gone. In some departments it's greed, I think they fund themselves a bit with police auctions. I'm not a conspiracy guy or anti-police, I have a lot of friends and family in law enforcement, but I do think it happens. Beyond that small percent of departments I just think the police don't have the man-hours to track down every owner of every recovered item or in this case to determine who is legally entitled to the firearm.
Ive never worried about it. 99% of units are opened before auction so facility can "inventory". So if there was a firearm....it's gone, you wouldn't know. Or, if they open it and you see a gun the price of the unit just jumped to a level beyond my willingness to pay because everyone is hoping for "hidden" firearms and/or ammo/accessories.
I think the likelihood of finding firearms these days are slim. With the shows it seems people are getting items out before they get behind. And more facilities have a clause about firearm storage, its a no-no. Lastly, renters insurance won't cover firearms in units anymore (at least mine won't)