"Years ago, I was pressed for time and stuck a perfect-appearing 22lr barrel in the safe without even a damp patch through the bore. Rushed off to work. Two days later it was severely rusted.  Badly pitted.  Not by some ancient ammo.  Just ordinary std. velocity stuff in common usage. Why? How? Not moisture, climate controlled and use desiccant packs...never a problem before or since.      
  I don't know"

Severely rusted in 2 days sounds suspicious to me!

I had an experience about 30 years ago where I wiped a solvent in a bore after a varmint shoot forgetting that the solvent had ammonia in it. 2 months later the barrel was completely beyond rusted when I took it out of the case.