Having been in a Navy aircraft squadron I know that the search for corrosion on metal parts is an ongoing issue with ALL aircraft. If they didn’t find it at the squadron level, it is because they were not instructed to look where it was found.
Exactly. It was not the CONCEPT of corrosion that surprised them. It is where it was and to what extent. Surprise! Engineers are not omnipotent when they design or write manuals.