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He is part of the royal household, but is serving away from the palace. Since he has told the family that his party was defeated by a "group" of robbers, no one suspects what this servant knows; Oedipus is the one who killed Laius and is presently the king of Thebes.
Huge coincidence. The lone survivor of the murderer is the same man who was ordered to leave the king's infant son to die on the mountain. He gave the child to the shepherd from Corinth instead . . . who is conveniently the same man who comes to deliver the news that Polybus has died. The Theban servant did not recognize Oedipus as "that child" during the murder, but when he returned to Thebes and saw Oedipus as king, he asked to be sent out to herd sheep again so that Oedipus wouldn't recognize and kill him, and so that he wouldn't have to accuse a king of murder.