How could he NOT know that he has a nail in his head?
Perhaps the whack with hammer had something to do with it?
The brain does not have pain receptors. Note from the XRay that it is quite a (heh) SMALL brain, as well. The nail seems to have stopped with the head at or slightly into the bone of the skull, so the skin probably healed around it uneventfully.
I wonder if he could have been radio-controlled, or if, walking by a 20 meter ham's antenna, he twitched and jerked anf flopped around for no apparant reason....A good case for a tinfoil hat.