Using an analogy of nomads being like robotic/computer guardians, how long is it before they go into some sort of "power saving hibernation" mode? The sleeper ships had jumped before the nomad fleet which blew up Sol arrived, and we know that they must hibernate because if they were created and active in the time of the Dam K'Vosh, why weren't they active in Sirius until the artifact was found?
The sleeper ships wouldn't have all arrived in Sirius instantly, and we don't know how long it took for them to cross the space between Sol and New York/London/Tokyo/Berlin, so the nomads could have easily assumed that the danger was over and gone back to sleep, thinking that all of humanity had died.
