I think I remember having one of those spots on a previous coach. It wasn't too noticeable, so I didn't do anything with it. My guess is it is on the inside, so you may want to pull it off and try cleaning the back side of it. I doubt that something is leaking from inside, but I wouldn't rule it out. If that doesn't work, you can always go pick up a roll of the new screw cover and pop it into place.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR