It is either one of those 2 bolts or one of the bolts from the plate behind the cam pulleys. There are no other bolts like that, except possibly the one that holds the dipstick to the alternator bracket, or the one that holes the harness bracket to the alternator. Due to the fact that the plating is not pristine in color, that bolt was probably not underneath the cover at any point. I imagine it has been jammed up in the back of the crank pulley and finally found it's way out.
