]the last place I would expect serious rust damage.
Up behind stuff is the first place things rust. You have been driving it in a salt belt for the last 8 years. Salt and road spray get everywhere and being up behind a heat shield actually makes it worse because when you go through car washes and get a under spray or try to wash it in the winter your self you cant get as good at the undercarriage or the spots at least that have heat shields.
Glad nothing bad happened.
This has happened a to a few people that I have seen on the forum.