I wonder if there is a way to filter for members of stack with no assigned stack markers. If not, this would be a nifty feature.
In the Filter by State, there is a Stack Details choice that contains: Stack Top Items plus all the individual stack markers that are defined in preferences. However, there is no obvious way to determine when stacks contain unmarked files. Another way to address this would be a change to the dropdown in Preferences - Catalog - Stacking to allow the choice of "None of the above" to allow one tag to be defined for images that don't fall into the other definitions for markers.
Currently when a stack contains an unmarked image, there is no way to view the thumbnail for that image. The only way to find such images are to open stacks and look for thumbnails that lack markers.