Reply To: Unknown character

Unfortunately you can’t delete them globally because text anchors share their code (U+FEFF) with index markers, tags, endnote delimiters, and several other things. You could try and see what goes on with this script and selectively delete them:
https://www.kahrel.plus.com/indesign/text_anchors.html
Or you can open the story editor, where you can see them better, and delete them in there. Do make a copy of your document, because you may be deleting more than is good for you.