How could he not know?? The damn this is nowhere near his hair line! "Say, what`s that silver thing sticking out of my forehead?"
It might have been under the skin. Remember, the skin extends further than the skull in an x-ray; soft tissue doesn't show up.
Look closely its countersunk. :)
I can tell you from experience. Several years ago a kid shot me in the top of the head with a high powered BB gun.
I was in my backyard bending over to pick soemthing up and the little bastard shot me!! You can imagine my anger. I bled for a while but didn't think anything more until I started getting sick. I was having choking like symptoms, couldn't swallow, just weird stuff for about six months. Then one day I was scratching my head and felt a bump, you guessed it, the BB. I removed it and the sickness ended.
So...to answer your question, you don't ALWAYS know.