This page is indexable and Snaglist found it through a link, but it is not in the XML sitemap. Google then only discovers it through links and visits it less often.
The sitemap is the list of pages you ask Google to index. A page that is missing from it still gets found, but later and less often. For paginated and filter pages that is fine. For a real page it is usually an oversight.
Snaglist reads the sitemaps of the site and compares them with every indexable HTML page it crawled. Every indexable page that answers with 200 and does not appear in a sitemap is listed, with how Snaglist found it. This check uses only the crawl itself, not Search Console. Without a sitemap, Snaglist does not report this.
Snaglist reports this as a notice. It costs 5 percent of the page in the health score. Good to know, and sometimes a deliberate choice.
In the report for your client this appears under the heading "Pages you do not submit to Google". When everything is in order, the report says: "Every indexable page is in the sitemap".
Snaglist checks every page of your site, for this issue and all the others. Start for free, no credit card needed.
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