From Halopedia, the Halo wiki
Adventurer is an achievement in Halo: The Master Chief Collection. It is awarded for activating the twenty-second Halo 3: ODST Audio log on Mombasa Streets. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.
The achievement is awarded when the player unlocks their twenty-second terminal, regardless of which terminal in the hub that is. For terminal locations, see here.