I had similar issues several generations ago. It turned out that the offending parts had dimensions of 0, so they didn't appear as objects in the model. I finally found them by selecting all the objects, and finding the stray hotspot without anything attached.
Peter- that did it! One part was found this way, the other was actually called something else, but the Flue.gsm was part of it. I still don't quite understand, but I've found the offending parts and replaced them with ones that work.