Looking at the log file on our CD server, I noticed that there are loads of warnings getting logged every millisecond for a document not found redirect for the home page of the scheduler website:
9376 09:20:17 WARN Request is redirected to document not found page. Requested url: /, User: sitecore\Anonymous, Website: scheduler
The scheduler is indeed a node of the sites section (this is a multi-site solution), third from the bottom and after all the actual public facing sites but has nothing more than name, domain and enable tracking set to false.
<site name="scheduler" enableTracking="false" domain="sitecore"/>
The only way I can even imagine the site getting accessed is by specifying via query string
sc_site=scheduler but that still doesn't explain the frequency that it is getting logged.