that's the only scenario that makes sense to me too JP.  either a rod
or main bolt worked loose.  That engine was holding good OP even at hot
idle; I didn't see any issue with it other than I would have felt
better putting new rings in it if I were going to keep it in my car
(but the while-you're-in-there's got me)  If it was just idling in the
driveway, any possibility of just changing the bearing shells in the
car and trying again?  might be the cheapest way out.  That was a NICE
looking crankshaft and it shouldn't have scored it if there wasn't a
load on it.  I have done in the car bearing replacements on cars with
far worse looking cranks and at least one of them is still being driven
daily (I know, it's not the RIGHT way, but if you have no $$ and a car
with a knocking rod, you do what you have to do.)

I do know that I only pulled one rod cap and one main cap so that
should help narrow your search, if that actually is the problem.  If
it's not I should count myself lucky as I was tooling down the road at
about 70 MPH just a couple hours before that engine ended up on the
stand!

nate