There was no dwarf traffic down there at the time, and it seems I had only breached a small cave that leads into a big floorless space. These "roof" areas end about two z:s down, where there's the actual caverns I guess. I see a part of the first floor, and some trogoldytes were having a mushroom party down there. I sealed off the small cave, since I'm afraid of flying fun stuff getting to my main stairwell. An FB can fly, right? So, I didn't have any access to the mushroom trees down there. Thieves seem unlikely, because they would absolutely be spotted by anyone of my 200+ dwarves, because of the narrow corridors and busy traffic (drinking------>retarded bin hauling------>staring at stuff-------->making pants------->drinking). So, what's going on? It must be a bug, I'd say. Any similar experiences? Any other ideas that haven't been yet discussed?