Just looked at bike. that'll be the chain (with sprocket) that has moved along the gearbox output shaft. Chances are, if this was a used bike when you purchased it, that it'll be an old scar, but it's woth checking the tightness of the two locking bolts (don't forget the threadlock if you do them up at any time).
Offer still stands for the replacement. U2u me if ness.