Originally Posted by ScottinAZ

back to the problem at hand, whatever it was (appears to be a bolt) was rubbing for a while. You can see where the threads on the bolt were. My guess, and thats all it is, is that a bolt was trapped between the primary and the frame, and vibration took its course. once enough clearance was made, it most likely fell out. This may well have happened long before you got the bike.