I know that someone will probably jump in and disagree, But I never pull bearing caps to just look. If the engine is that far apart already and you found metal where it shouldn't be, I'd rebuild it. For me, "patch" jobs never work and cost twice in time and parts. If I had to guess, I'd say that what you found came from the pitting you see in the bearing, but it could be a collection from all the bearings. I think it would be wise to check the cam bearings as well. Good luck!

Les