Much of the credibility in action/adventure games comes from the ability of NPCs to properly detect the player and its actions in the world. If the NPC detects things that it shouldn't or misses obvious stimuli, immersion and even entire gameplay systems may be broken. What is needed is a robust, designer-configurable sensory system to provide believable detection and reactions from game agents. Drawing on work from the 'Assassin's Creed' and 'Thief' franchises, this session describes how to construct advanced sensory systems to provide gameplay depth for the player. The presentation focuses mostly on visual detection but also covers audio detection.