it kinda looks like smut build up on the glass to me. take the door off, go outside, spray oven cleaner on the glass, let it sit for 5 mins, and scrape it off with a scraper. clean the surface with cooktop glass cleaner.
I bought the Klein MM1000 to check the electric in my garage + a few breaker which were dead the preowner or who else put up the pegboards in the garage was so fuking stupid. 2 dead breaker, half of the guest bathroom with no electric. nails and wire don't chime together. pics will follow in my new garage rehab post in the next days