Some bones and skulls are placed outside of the playable area, either behind the animal den wall or below the cave floor. I discovered this after a few of them appeared on the bridge headed up towards High Hrothgar just northwest of Ivarstead.
I think I found all of the references that are out of bounds, though it's possible there are more hidden underneath the cave floor somewhere since there are a lot of placed objects in this particular troll cave.
000CED3C, 000CED3D, 000CED3F, 000CED43, 000CED44, 000CED5C: These are fully buried and out of bounds.
000CED60: This troll skull is partially buried.