Duct work, bad choice but for very short term.
I recently refinished a Smith .22 auto that had been left in a duct for a while.
Ther wasn’t just rust but the aluminum frame was pitted and the anodized finish was heavily damaged.
The question becomes what are we hiding the item or items from?
Door to door confiscation? Better not have “X” on the property.
Petty theft? Hiding may work, and it may not.
Fire? Better be ready to lose everything on premises that isn’t in a fire proof safe.
A number of years ago I helped a couple retrofit a couple of file cabinets with hidden storage. Their employer didn’t allow firearms or knives bigger than a pen knife. Yes they could have found different jobs but at $150k a year each, they decided to tough it out.
They redecorated his office, including two very nice file cabinets, that contained a BOB each, broken down to fit in the hidden recesses, and a handgun each. Just enough to get home (1.5 miles) or to a cache elsewhere.
The older I get, the more I like off site caches for many items.
Firearms without paperwork and properly packaged (no fingerprints) are one item that can be placed and forgotten about until needed. Spare clothing, food and such also, and not just BDUs but nice clothing, in case of fire for example you can still have decent clothes to go to work in without having to replace your wardrobe immediately.
Documents are one sticking point.
Photocopies are dangerous, with them anyone can steal your identity, if they are found.
Memory sticks have their issues also, such as a power outage and all your info is on the stick? May as well not exist until you can get that data off the stick. And again, if the stick gets stolen, best hope the encryption is better than good.