George,
I'm thinking it's the guns themselves plus plain old air, not the cases. What was the make of gun?
That the cases were left open is telling. If it was the felt or felt glue, then I'd think the side of the gun touching the case would be affected much more than the side of the gun exposed to open air.
I can't think of anything that would outgas as an acid. Organics (solvents) outgas. Formaldehyde isn't going to be acidic and it's a common glue residue.
If the plastic case was PVC I truly doubt that it could degrade sufficiently to release HCl gas, which PVC will do when burned.
