Toxaris' word add in has a Mark feature, it can colour highlight words defined in a list, if I don't remove the highlighting before I save the DOCX and convert to EPUB I get ugly spans sprinkled though my EPUB.
The Dialogue Check feature of the add-in uses a tagging technique - at the end of the dialogue check the 'tags' are removed. This is a commonly used technique in Word macro's and add-ins. I use it directly in Word, Sigil, Notepad++ etc, there's probably a name for it - 'false flag substitution' springs to mind
If the 'false flags' are left in place then they'll end up in the saved DOCX and hence in an EPUB created from the DOCX.
If I leave bookmarks in a DOCX, I get ugly spans sprinkled through an EPUB I create from it.
I have a Word macro that runs when a DOCX is opened, and another when its closed. I could include code in those macros that would inject text into the DOCX that would result in ugly spans in a subsequent EPUB.
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Roger - check
all the macro's, you'll probably need to enable the Developer tab to see them all, maybe your Word has some dodgy macros installed. I've seen it happen with custom Excel installs in the corporate world. Look for that Policepar... string.
BR