It took shining a flashlight in the corners, but I do see it. #12913, built 11/03.

Garage kept, washed weekly (at least), ~5800 miles, but it does sit outside in the elements while parked at work 5 days a week.

Whatever it is, it's not on the outer surface of the glass, so I doubt washing has anything to do with it. I tried to 'feel' up under the lip of the glass, and looked under with a flashlight, nothing immediately obvious.