The brake ducts could have been picking up random particulates and heating them up. If you had sparks flying from brakes A you would have known. B it would show somewhere
As long as you checked it (assuming you know what to look for, it seems you do and brakes really arent complicated) and nothing is wrong. Check for other things wearing or running/ scrapping.
Other then that enjoy! How was Watkins glen? Was it the full track?