A lot of guys will store their knives in the sheath for long periods which can lead to corrosion depending on what the leather was tanned with. I see handguns that guys store in the holster all the time and they usually have corrosion from the chemicals in the holster. The other thing I see as a gunsmith are large numbers of nice old shotguns that have been stored in a case lined with sheepskin and slid under the bed for years. The highest humidity in most homes is down at floor level. The sheepskin sucks up and holds that moisture like a sponge. I have seen beautiful vintage shotguns ruined by people doing this and they have absolutely no idea why it happened. The best way to store guns and knives is in open air in a rack, on a shelf, or in a drawer with some oil on the steel - NEVER in a case or holster or sheath or a cardboard box. Those things are only meant for transport or carry - not storage.