If an object (say a box) is on top of an object/Rug that is on another object/Carpet, it seems a npc cannot see the box.
Also, worth noting I had some alternative descriptions set to npc must have seen object but it didnt seem to pass until i changed it to Object is visible to character...
Whether or not one of these "have seen" or is "visible" is the preferred method for a restriction check in this example i'm not sure.
Object ontop of several objects cant be seen by NPCs
