Ok so it wasn’t physically traced back to its source then…
Assuming it is legit and supposed to be there, the surfaces should still be cleaned and the connection bolted tight so it can do its job properly.
What’s to know? The bolt looks loose, it is a ground I’d say. I saw a few like this on my brother’s 96. Did you chase it back?
I’d unbolt, clean everything to bright, re-install, and consider coating with dielectric grease or fluid film.