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 For instance, a link to a forum page (which was created by a forum plugin I no longer use), and links to unpublished posts and empty tags.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?