semrush.com
Observation of server logs revealed that semrush appear to parse comments in HTML code. The consequence of which is that hidden URLs are potentially indexed and are accessible for analysis by their algorithms.
The outcome of this is that requests for the URLs that don't appear to a regular web browser are made. If these URLs do not exist a 404 error will appear in the logs.
Looking back on this page I am not sure why I was looking at SEMRush with respect to William Stern and Stirnet. Stirnet, using Wordpress Memberpress, does do some strange things!
SEO and other services
well_known_traffic_advice.htmlStirnet
My page on William Stern had a commented out reference ../bb4fz/burgoyne1.php#dau6.