Old links still showing on Sitemap, creating 404s

I’ve been checking the 404 errors on Google Webmaster Tools and found that some of them were linked to by my sitemap. For instance, a link to a forum page (which was created by a forum plugin I no longer use), and links to unpublished posts and empty tags.

I checked, and the sitemap showing on Webmaster Tools is current, and I don’t have any old sitemaps listed. So I assume these links must exist somewhere, even though I can’t find them anywhere on my dashboard (even in the Trash).

I’ve checked on cPanel and I can’t find any old folders or files left behind by the old plugin.

Can someone give me some clues where else I could look?