Some device, at some point used that subnet. People remoting in from wifi, home, etc would be the most typical. Big issue with this is that you can't trace it back to a sensor as there is no detection vs sensor relationship, just sensor to covered subnet. Since covered subnets can be by more that one sensor you can only have a best guess!
Hope this helps.
That was my guess as well; but I am still new to RSD, so the real question is if I delete all of the uncovered subnets from ePO will I cause a loss of functionallity in systems that are on those subnets? I have sensors deployed to all of the DHCP servers; so my hope in deleting them is that the DHCP sensor will pick up these systems/subnets when the agent checks in. Please tell me if I am way off base with this or will they just repopulate because I haven't cleaned up the db?
Nothing loss, they'll just show up when they are active, on their next broadcast or dhcp request...