Attached pics are looking up at sub floor in basement. Yes, I know what it looks likes but it’s an area that’s surrounded by completely dry sub floor with dining room right above it. Tramex reading was only 11 at it’s highest on the darkest stains. No odor after scraping a bit with my finger and it’s as hard to wash off as motor oil. Any ideas? 16 year old home with no signs of moisture intrusion anywhere.