One Got Away Achievement in Halo: The Master Chief Collection

This Easter Egg can be found in the Epilogue. 

During the Epilogue’s cut scene, Sergeant Johnson and Dare will begin talking about ONI and the Engineers. Then, Johnson will sit down with the Engineer that was extracted during Coastal Highway and hand the alien a cigarette lighter. While this happens, pull left on both analog sticks and the camera should pan left to a frame of Buck awkwardly touching a caveman, similar to the those found in Halo 3’s “Sierra 117” mission. The achievement will unlock once you’ve spotted this strange frame and the cut scene concludes.