A crack!!
I once showed my wife a picture of a crack in one of our helicopters and she went white and expressed her shock/horror. I then put her straight......
"Darling, its a helicopter it therefore has a crack somewhere - that's what helicopters do, they vibrate and they crack. Don't worry about it.
I'm not sure how reassured she was but it's close to reality for almost every type I know - look at the doublers on the rear fuselage of any machine more than a year old.
G.