In the mission where you need to track down the Penguin on his ship (it is one of the first main missions in the game), after your encounter with Tracey (Penguin's assistant) and her goons, you will reach the first predator room in the game. Stay crouched, sneak up to an isolated enemy, and press Y to perform a silent takedown. Do this to all six enemies in the room, without being noticed, to get the "Anyone See That?" and "Silent Knight" achievements. Note: If you have already passed this point in the game, there are still plenty of other opportunities to get these achievements, but you should definitely do this before completing the story. After completing a predator room, the enemies will not respawn.