I've had as similar problem.
I had two depots (about 10 squares apart, one inside, and one outside).
So the humans came along. and started occupying both of the depots. (one of which was inaccessable)
I tabbed out to chat in #bay12games
about a minute later, all of the merchents, guards, and liason were gone.
but all their items were there, fresh for the picking. took me nearly a season to haul it all in. (I only had 10 dwarves at the time).
The came and went regularly next season. they didn't seem to mind buying back all that large armor and weapons.