tpack on "[Plugin: Wordfence Security] googlebot.com reports 'tried to access non-existent page'"

Wordfence Live Traffic reports:

Mountain View, United States tried to access non-existent page http://mydomain.com/sitemap.xml
IP: 66.249.85.160 [block]  Hostname: google-proxy-66-249-85-160.google.com

I use Yoast's WordPress SEO plugin to generate sitemap file. I can open and view sitemap.xml in browser, but I keep seeing it being reported as a non-existent page. Goggle webmaster's page also shows that all pages listed in the sitemap.xml are indexed.

Any ideas why it is being reported as a non-existent page?