Milk Carton Achievement in Halo: The Master Chief Collection

This Easter Egg can be found in the mission, The Storm.

Towards the end of this mission, as you enter the final outdoor area that leads to the Anit-Aircraft Battery, make your way through the Covenant forces until you reach the Prophet of Truth’s hologram console. From here, turn to your left and you will see a narrow corridor with small storage rooms on either side. The poster of the missing man can be found lying on the floor in one of these storage units on the left. When you find it, simply zoom in on the poster and the achievement will unlock.