Partway through Chapter 5 you will reach the part of the turret production facility responsible for quality assurance. All the turrets going past the office on the production line will be scanned and compared to a fully functioning model lock in a little room over looking the production line. Wheatley will very kindly hack the door for you allowing you to gain entry. When inside remove the turret from its location and stand in its place until you yourself are scanned. Trophy unlocked