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On Chapter 3 (Test Chamber 16) there’s a turret tucked away behind a metal railing at the start of the room (on the right). Use the portals and the laser beam to kill the three turrets as per usual, then use a portal to get the beam crossing near the door. Use the reflection cube to kill the turret, opening up the vent. Crawl inside and look down for an impromptu X-Factor audition!