Al Jazeera has a terrific collection of Space Shuttle photos. In photo #11 we can see something bright in each Space Shuttle Main Engine (SSME) as Endeavour lands. What might that be? A reflection off the back of the combustion chamber? Something related to the auxiliary power units (APUs) venting through the SSMEs?
Here's a smaller cropped version; see the "photo #11" link for the original.