When your character wants to make an active attempt to study their surroundings, notice clues in a crime scene, or search for hidden foes, they use the Perception skill to do so. Perception represents your character's conscious use of all five senses to observe their environment (or, if you're playing some sort of alien species, your character may have more than five). In any case, the most important point is that Perception represents your character's active use of these senses.