Observant

Observant is an achievement in Halo: The Master Chief Collection. It is awarded for activating the twentieth Halo 3: ODST Audio log on Mombasa Streets. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.

Location
The achievement is awarded when the player unlocks their twentieth terminal, regardless of which terminal in the hub that is. For terminal locations, see here.