You mean that was in the crank case? Or in the oil drain bin? If it were in the cylinder then the piston would have struck it and put an end to that engine.

Normally when a bolt breaks it happens at the head. This is broken outside the tread. This part should been stuck into something. And the head part should fall to bottom of crankcase.  No marks from any abuse. So my guess is it has been there since the engine was built. The cylinder head cover has M6, maybe there is some M6 inside the clutch cover. Transmission? Most bolt are outside crank case, were the oil is.