URL reference in TOC cannot be resolved

See image above, I don't know if anyone can help me with this. I am generating MS HTML Help. The page referred to used to be a very long one, with 10 bookmarks, all having a hyperlinks to them at the top of the page. I've broken the content down into separate pages, and the bookmarks are obsolete. In fact I've removed the old file from the project, as well as (I thought) having removed the TOC references. It's possible that these old bookmarks are linked somewhere else but I can't find them if that's the case. Can someone help me identify what file or where I find this bogus link?
