I'd like to add that this is the case only for "rare" items. Where as normal backpacks, foodstuffs and ammo stay in the tents, it's scopes and high capacity vests that vanish.
Me and a friend are playing on a persistance on server, and have 2 tents set up. I've left a mosin with scope outside the tent, near it on the floor. And have a PU scope in the tent. We played last night, and left the gear. I logged on this morning, the tents and most of the standard items were there, but both the longrange scope and PU scope are gone from tent and rifle.
So, even though persistance "works" for the bigger part, I suggest not logging off from servers with rare items in tents or outside. Just have them on your character instead.