Twice I have seen items appearing on/in walls. One was a rare card, when working on the L10 rat dungeon. It remained over several attempts, and at least one re-visit after the level was cleared. The other was a Guardian Sword in the Bastion (not at home so don't have the device with my game on it). Both times the item was in the upper left corner of the map, on top of the graphic of the brick walls, diagonal from any path I could take.
I will try for a screenshot if I get back there, or if it happens again.