Not exactly gun cases- but a tip if you like. I've purchased several used shotguns over the years, M12's, Ithaca, Fox, Smith with recoil pads. I got in the habit of removing them, just for kicks I guess, as the M12 3" Mag had a shorter stock lop, and the older solid red factory pad- I wanted it a tad longer, and when I shone a pencil flashlight into the bored bolt access stock hole, Lo and Behold, I fished out a crumpled up $50 bill- Grant and all. I also removed the lead weight, as I shoot somewhat better with muzzle heavy shotguns, at least on waterfowl.
I bought a Swiss Schmidt-Rubin, and when I removed the steel buttplate, a Swiss Military ID card fell out, and a folded letter to, I assume, his family-- dated 1945, and in German.
Best story, no firearms related, but Mae and I were in Nashville, at Gruen's Guitar shop, where I bought a second hand Gibson Sunburst acoustic guitar-with battered case. When we got home, I opened the storage department, set of Black Diamond strings, a few hardpicks, a capo, and a folded up envelope-- 2 clear plastic baggies, partially fill of white sugar-like powder, a union card, and $300 in bills- sure helped to pay for that old Gibby- I flushed the white powder down the toilet. And as I paid for the guitar in cash folding money, no trace to me in case the former owner came back to retrieve in-or st least the case. RWTF
