Obviously there are other ways to insert an 'invisible' comment or the like:
<?xyz?>
Why this!? Any distinctive features?
<?dp n="579" folio="579" ?> For what is such an entry good? (I guess and anticipate one of the possible answers: Ask the publisher!)
The example is from a very serious book.
EDIT n="579" folio="579 and so on are most likely unvisible page numbers.