AP article
A law enforcement official says investigators have found a bullet aboard a US Airways plane that was grounded in Charlotte, N.C., after a pilot making a pre-flight inspection found a hole in the fuselage.
The official says the location of the hole found above a window in the plane suggests that a bullet came in from above, not from below.
That was one sharp-eyed pilot, and certainly answers the question of whether pilots ever notice anything on their walkarounds.