The "P" in Fallout's S.P.E.C.I.A.L. stands for Perception. This is the measure of how aware the player character is of his or her surroundings as well as how well he or she can use V.A.T.S. Unlike in previous games, this stat does not impact when enemy NPCs show up on the compass, as they don't appear at all in Fallout 4 unless they are actively fighting or searching for the Sole Survivor.