Basil........A borescope plug being left out!....but those items are on the independent inspectors list of things to sign off....how did that happen?
Having said that, I know it does, because exactly the same happened to me in a B747-200 out of Lagos heading for JFK, and let me assure you, we didn't really want to turn around and go back, but after a fire warning and shutdown, there was no option.
Still, it never ceases to amaze me how it can happen...
Cheers