I do not have an answer to your question, but this page does list all of the different Structure Pane icons: InDesign Help | Structuring documents for XML. Yours refers to an empty element (empty frame). I couldn't easily recreate it in a sample doc, so it's not super helpful on its own (but that page is kind of handy).
Thanks for the link, yes, it will be handy.
One thing I've discovered since I posted is that selecting that element's parent in the Structure panel and Deleting it with the trash can icon does get rid of the problematic one. Prior to using the trash can I was simply un-tagging the parent, which just moved the problem child (I love puns) up one parent. So that at least keeps me from having to re-tag the whole frame.
OK (he says with a blush) I've got it, noticed a commonality I hadn't before posting.
The page to which cchimi kindly supplied a link describes the icon as an empty element and that it's attached to an empty frame. However, it isn't only an empty frame.
What happened in our file is that the script tagged an anchor marker with an xml tag, which InD apparently doesn't like very much. Kind of makes sense, really.
Now, I maybe cracking up, but with one instance of the "bad" element I THOUGHT I was able to see the empty element in the story view with the anchor just a few characters away. I selected the anchor and dragged it back in between the start and end code and the element's icon changed and it came "alive" again, I was able to manipulate it. For most of the other troublesome elements, though, AND sinpy tagging an anchor marker, the XML tag does NOT show in story view. The element gets the empty icon when you simply tag an anchor marker, in this case it is an anchor for a rule.
Hope this might help somebody, some day.
Thanks again for reading.