Lurkers

The Lurkers are repeat encounters and sightings of dark vessels at the edge of the interstellar frontiers and observable space. The moniker referred to rare interstellar phenomenon associated with suspected extraterrestrial intelligent life, usually after humanity achieved spacefaring status with the advent of the Shaw-Fujikawa Translight Engine and Slipstream FTL travel. While a general and nebulous term, it fell out of favor following first contact with the Covenant empire and exposure to ancient history from time before the Halo event.
However, a fraction of strange sightings and encounters of suspect alien activity cannot be attributed to Covenant and their own fringe extraterrestrial encounter experiences/knowledge. Some of these sightings and hypothesis fall under the "Meddlers" theory, civilizations acting and observing from the periphery of human/Covenant regions but maintaining mostly peaceful noncontact.
Lurkers imply something other. Not peaceful, not observing; hiding and plotting. The term implies a faction, or factions, operating at the edge of known space with more malevolent agendas but such plans remain shrouded like the Lurkers. The concept perpetuates a cynical worldview of galactic interactions between species, most by humans, but what if the fear is justified even for a moment? "Who's watching the watchers?"