Hi Gail.
Yellow lines indicate that the link is from an image map. If
the bookmark is mapped, the link view should show something like
"topic1#bookmark1". The bookmark is referenced by the # character.
If you can see this, the other end of the yellow line indicates the
topic that contains the image map that has the link to the
bookmark.
Unused bookmarks cause absolutely no problems at all. It's a
bit like the spare tire in your car boot. If you delete a bookmark
that is linked and save your project, you'll see a Broken Link in
your Project tab.