Yeh .. I've been working on a Cartoon and Anime tool-kit (should be with Daz very shortly). I found these hidden lines occur as a result of certain props. For instance, working with Koshini, I found her hair causes hidden lines to show up in sketch renders. Just making the prop invisible solves the problem. Some clothes have the same effect, but not all. The answer is messy, but works ... do two renders. One with everything visible and one with the offending prop (or props) invisible. Then tidy up your lines in post work. Why do some props cause this to happen and others don't? Haven't a clue. I've been through the files carefully to find the anomaly, without success. Bizarre.