Check the containers in your characters' inventories (there are pouch containers that have the same icon as a pouch of bone dust, etc). If you have gold in a container (possibly a crate picked up without looting first, for example) and a character's inventory, if they open the trade window, it will all be moved to the container. Gold and other stackable items can also be put into containers when transferring items between characters (handy if you always want your archer to put arrows in a backpack, but somewhat less useful with gold).
