I keep seeing the below pop up in my logs. It's for artwork from Jakub Steiner, an artist who created a lot of gnome artwork (icons, etc.).
Sender: gnome-software Message: not handling error download-failed for action refine: failed to download http://people.gnome.org/~jimmac/gnome-software/slack-logo.svg: Not Found: <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>404 Not Found</title> </head><body> <h1>Not Found</h1> <p>The requested URL was not found on this server.</p> </body></html>
That URL is a Red Hat Enterprise Linux Test Page that's gone 404.
sudo grep -rnw '/' -e 'http://people.gnome.org/~jimmac/gnome-software/slack-logo.svg'
... to search files for that text string, I find a lot of /var/log/syslog and /var/log/auth.log entries.