How come Gandalf managed to get Thrain's key in Dol Guldur ? How was Thrain able to hide it from the Necromancer ?
It's pretty easy (if uncomfortable) to imagine how Thrain hid the key. I think the better question is how he concealed the map. It was written on parchment and I doubt that he was able to hide it in an orifice. More likely that it was tightly folded and sewn into a piece of clothing. Perhaps it was later transferred to a crevice within Thrain's cell (along with the key). 'There are older and fouler things than Orcs in the deep places of the world.' - Gandalf the Grey, The Fellowship of the Ring