If you get this warning, a URL was found in your website but the plugin couldn’t find this URL in either the database or in one of your files.
In some cases, the URL will tell you something about the origin so take that URL, open your site in your web browser, right-click, and click “View source”. Then do a text search for this URL. In most cases this should show you where it’s inserted, and may tell you what to do about it.
If you can’t figure it out, please contact me so I can help you with this.