You can get this trophy anytime after you get your first gun. To get this trophy make sure your gun is equipped (if not just hold R2 and use left or right on the D pad to equip the gun). Sneak up behind an enemy (this is easiest to do if you press X to crouch first), once behind them press X again to stand up if you're crouching and hold L1 to aim at them. Aim at the back of their head, Snake should say "Freeze!" and the enemy will drop their gun and lie on their stomach or raise their hands above their head. Then you need to release L1, stand near the enemy and press triangle. Snake will start to check them for items. Press triangle again any time you see ! flash up on the screen and after a few times Snake will find an item and you will get your trophy.
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