You need to “drop” all collectibles on nearest stable square. Are these items “pick-up-able” by anyone, perhaps in the explore mode? Or can they be “stolen” in the usual way, even though they are not in a player’s possession? If there is no way to recover these items, are they not effectively discarded, and shouldn’t they be removed from the board?

If something is sitting on the board it can be picked up as usual and is no longer considered as being held by any player.