Hi Danny, yeah I think you guys have figured it all out, here is that problem surface:



The trimming curves are totally on the outside of the underlying surface...

Because all the edges are joined to other edges, it basically still counts as a solid, although with a really bad tolerance since the gap between those edges and the surface is substantial.

The bug is in whatever made that particular surface in the first place which is a bit hard to determine now. If you happen to know which modeling step that opening appeared in (like after a boolean, after an initial extrusion, after a certain step) then that could help to narrow down which operation has the bug in it that created that bad surface.

- Michael