This only seems to have started happening since I went from ePO 4 to 4.5 (not migrated but a fresh install)
We see this on our 4.0, however, when you look under Detected System Properties or Related System Properties it will populate with usernames.
4.5 Agent seems to do that part.
Best bet is to make a query for Managed Systems and search for Users under Computer Properties, Detected Properties, and also local user too.
Correlate the N/A machines with Event Logs and force a Wake Up on them to make sure no one is logged in. Document and Settings will tell you which user profile was last modified and you can check that against the custom query.