There are more Woodruff keys inside the crank cases or under the covers that could end up in the sump. Primary gear and oil pump gear on the left. Timing shaft on the right.
Aren't these ones smaller? - I can't remember exactly which sizes are where, but there are definitely two different sizes used.
I would say it's more likely a 'spare' that fell into the engine whilst being worked on. It's possible that someone lost it due to not locating properly during assembly and so it is missing from somewhere but I don't see how it could have come out of a fitted part without causing some serious damage. Could it be from the alternator? if that was torqued up properly it would likely still run OK without it plus, it's fairly easy to check without splitting the engine. I bought an ST2 engine as a donor and found the shim/washer from behind the flywheel, in pieces in the sump but the flywheel was still tight due to enough of the washer being in place, not sure how long it would have run for though if left?
