Monitoring candidates is the same thing as "Requests to unmonitored hosts" actually. These are hosts/processes/services that are interacting with something in your environment with an agent on it, even though the unmonitored host doesn't have an agent on it.
So I expect that it will appear as a monitoring candidate. Now I'm not aware that there are systems which doesn't have a OneAgent on it. What is the reason that it doesn't show up as a Monitoring Candidate?