Lessee now... Could be any of the following
A. Geosetanims
B. Mirrored Verticies (IE: The skin is showing up in the inside of those verticies instead of the outside, I had this happan with the turtle shell a few days ago*
C. The skin
If the part appears there in Viewer, and is on the same geoset as the rest of the body, I blame it that the skin is showing from inside of the model. To check for this, scale the model up in WE to about 5 or 10 times, then put the camera inside it. If you can see the skin, thats evidence that the verticies have had the side with their textures flipped
Got no idea on how to fix it though, sorry