This has happened to me twice and I do not know if it is intended behavior or a bug. Twice I have unloaded items in the insurgent stashes on the island at the south-west of the map, I then left gathered more items and returned to find the entire stash box had disappeared. Oddly a single weapon was on the floor near where the stash was located.
Is this a bug that I triggered by manually dumping items into the stash or do they despawn after use?