Several years ago, game time, the human caravan stopped showing up. I didn't think anything of it, because stuff happens in the world sometimes. However, about two years after the last human caravan, I suddenly got a series of announcements telling me that a black bear had guzzled about 200 units of booze. No booze of mine was outside the fortress. I zoomed to the spot and found a black bear unconscious and dying of alcohol poisoning at the gate of the defensive wall.
After the most recent dwarven caravan left, I found some wagon logs and a pile of goods at the same location at the gate. Apparently a wagon was scuttled there. It had to be the most recent caravan, because two grazing animals in cages were still alive in the pile.
There's nothing visible at that spot, or anywhere else on the map that looks unusual. No combat reports or announcements were provided to me. I have a hypothesis now that the last human caravan, or part of it, is somehow stuck in limbo, and that's why the caravan stopped showing up (because the game thinks one still exists); the black bear stole invisible-to-me liquor from the invisible-to-me wagon. The dwarven caravan narrowly escaped this fate by scuttling one of its wagons.
Is this something that has happened to anyone else? What could be going on?