FAIRadiusToRunDetectionInterior
Distance from the player to other actors (NPCs, creatures) at which the game starts checking whether the actor notices the player. For hostile actors, this is the distance where the actor may notice the player and attack, assuming the player is not hidden in some way. However, hostile actors are also controlled by fHostileActorInteriorDistance. If fHostileActorInteriorDistance is less than FAIRadiusToRunDetectionInterior then the hostile actor may become combat ready but will not attack until the player moves closer (unconfirmed). NPCs in this situation will taunt/threaten the player.
Increasing this value makes dungeon exploration a lot more dangerous.
FAIRadiusToRunDetectionInterior only applies in dungeons and buildings. See FAIRadiusToRunDetectionExterior for detection in the wilderness.