I have heard of shit disappearing, but I keep dumping random shit (like a million iron daggers) on the ground inside of a town (Whiterun) and haven't seen any of it vanish. Just don't store it in a dungeon because when a new quest brings you through it, all the chests/items/mobs are wiped & refreshed.
edit: ^ yeah, I think you can rely on any chest that you are granted legal access to. For example, befriend certain people or earn certain authority in places and you'll no longer be considering "stealing" when you take their stuff, so the chests and whatnot are free to use. However, I raided and stolen everything in a shop before and found that they restocked all the shelves/counters/displays/strongbox later, so beware of that.
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