It's not there in the 10.4.1 server I have access to either. I've seen weird logs when an invalid service name is accessed, but I don't see anything in the log you're looking at. Are you doing any administration in ArcMap? I ask because ArcMap makes SOAP calls to ArcGIS Server and these days would be the most common client to make a SOAP request.
May 30, 2018, 2:54:02 PM Error handling request to service 'System/RasterProcessing.ImageServer'. The service 'System/RasterProcessing.ImageServer' was not found. Verify the service exists and is started. Soap
That said, I tested with ArcMap 10.6 against my 10.4.1 server, and don't see any errors like that. With that said, it looks like some client somewhere is trying to make a request to a system service that doesn't exist on this resource. Portal only makes restful calls...
Might be able to take a look at the web tier logs and correlate the time stamps and find the IP of the client that's reaching out to your server(?)