When you do a search on various criteria, it may happen that files which correspond exactly with the criteria are in the same stack (or version set) as others which don't "match" fully those criteria. Those files are "non-match" search results and as the stack is expanded, those are marked with the red circle with diagonal line. In other words, the search has found the correct stack, but not all the members of the stack are fully matching the criteria.
By the way, I have not found anything about that feature in the online help docs. I don't know if there is somewhere a doc about the various icons on the thumbnails for :