Instructions
Fight through the courtyard infested with grunts during the "Mombasa Streets" mission. Once the enemies have been defeated, cross the courtyard and find a door marked "Detour" to enter Tayari Plaza.
Walk across Tayari Plaza. The plaza will be covered with bodies, indicating some previous battle. Enter the hallway at the west side of the Plaza and proceed upstairs.
Follow the main hallway until it leads upstairs to a third level. Walk down the hallway on the third floor of the building. As you make your way to the end of the hallway, an automatic door will open and reveal two Covenant snipers. Defeat them and proceed up the stairs to the fourth floor.
Walk down the hallway on the fourth floor until it leads you into a room featuring a helmet smashed through a display screen on the left wall. Walk up to the helmet and press the "RB" button to inspect the helmet. The screen will fade as the Tayari Plaza flashback mission begins. You will be rewarded the "Junior Detective" achievement for finding "the first clue to unravel the mystery."