Patterned HW staining, guesses on how it got there?

I don’t have much time now when I have more time I can upload more pics and go into detail of the backdrop to the story. I’m asking for a friend. I could contact some guys directly but thought I’d throw this out there.

Suggestions? It looks like stage 2 hw corrosion to me, but it’s in a Cresent shape on the glass 2 inches wide