"Marked by the Zone" is not the people, not mutants. Their appearance and spiritual world greatly changed. It is, as a rule, "initiated into the mysteries of the Zone." Some believe that they manage all processes of the Zone, mutants, anomalies, and even something more global. If they don't like some Stalker or even a mutant, they are unable to punish him, or opposite to, for example, help Stalker to get out of the maze of anomalies and aim at the artifact location, or spend a little blind puppy to his pack. Someone hates them, and someone admires them. One they help always, others only in critical situations. Say that those who lived here before the first explosion in 86, that's only not people, they are more something else entirely. They are a mystery Zone. They never go out with anyone for a direct contact, only a flicker among the ruins and trees like ghosts. Might have telepathy as met them and talked about the voices in his head that led them. Exactly what their appearance, no one can not describe, but people were talking about the dark silhouettes of the coats with some colored lights instead of eyes. As they say experienced stalkers marked Area are everywhere, they the Area.