I have created several stashes deep in the woods near the southeast coast, usually placing backpacks and an occasional rifle on the ground beneath dense fir trees, to provide cover.
Several times, I have returned to the stash after a short time to find the backpacks have vanished. Each time, it was on a server with 3 or fewer other players, and although it is not impossible that the stuff got stolen, I am reasonably sure the stash was not discovered. Three incidents happened on two different servers. Backpacks included Improvised, Taloon, drybag and Hunter.
In one instance, I logged out and returned; in one instance there was a server reset.
Considering that there is also a problem with dropped vests disappearing after a short while, there needs to be a clearer explanation of what items are intended to persist, and which ones will despawn over time.