We redeveloped a site in 2012 that had been a simple HTML page catalog site. It became a dynamic, SOLR-driven site under the same domain name. The site content is updated daily (although not all products change daily) and there are now nearly 300,000 products. XML sitemaps of the new site are uploaded daily. About 2 years ago we moved the site to HTTPS.
Google Webmaster is still reporting pages from the old site (blah-blah.html) in the the Crawl Errors under Not Found. All the pages are showing status 410. Here's the relevant part of the vhosts
RewriteCond %{REQUEST_URI} ^(.*).html$
RewriteRule .* - [G,NC]
We cannot see where Google is finding these dead pages after 6 years! We remove them in Webmaster and mark them as fixed but they keep coming back.
How long do we have to wait before Google stops 'finding' them? How can we find where Google may be finding old backlinks - there aren't any showing in search?