James Webb underwent a very specific cool down procedure. It included a phase where everything but the instruments were cooled down first, so that water from e

James Webb underwent a very specific cool down procedure. It included a phase where everything but the instruments were cooled down first, so that water from elsewhere in the spacecraft didn't stick to the instruments. Moving a bit into speculation territory, I can make a more specific guess. The article for Euclid specifically mentions insulation as the source of the water. It appears that the water in the other components were properly accounted for in Euclid's cool down procedure. For Euclid, insulation directly surrounds the telescope portion. James Webb is build different. There's insulation on the solar panel side, but the giant heat shield is between that and the mirrors and sensors. I don't think there is any insulation on the mirror and sensor side, because besides the sun there's no significant sources of heat where James Webb is.