From Halopedia, the Halo wiki
Examiner is an achievement in Halo: The Master Chief Collection. It is awarded for activating the third Halo 2: Anniversary Terminal on Metropolis. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.
The terminal is found on the first floor of the building the player enters to make their way up to Sgt. Banks. Upon entering the building it is on the left wall behind some crates, and takes the form of a monitor flashing "INCOMING MESSAGE".