From Halopedia, the Halo wiki
Supply caches were secured rooms in the city of New Mombasa, filled with supplies such as weapons, ammunition and in some cases, Mongooses, presumably stored by the city's police department or the UNSC for emergency situations. During the Battle of Mombasa on October 20, 2552, the city's Superintendent unlocked several supply caches for a lone ODST in order to help him survive in the Covenant-occupied city.
Halo 3: ODST
Supply caches appear as a feature in Halo 3: ODST and serve as a primary source of human weapons and vehicles in the hub level Mombasa Streets. They are unlocked by finding Audio Logs. There are 7 available supply caches. The supply caches are marked on the map with the Superintendent icon. The interior, and sometimes the immediate proximity, of a Supply Cache is marked with symbols used by the Huragok, the center one of which usually depicts the Superintendent's avatar. To open the door, the Rookie must stand in front of the door's camera. The Superintendent will then open the door for the Rookie if he has unlocked this cache by finding that cache' needed Audio Log.
The supply caches contain enough weapons to supply up to 4 players from empty. All six supply caches contain several identical items:
While two of each cache will contain one of the following:
It should be noted that the M6C/SOCOM and M7S Caseless Submachine Gun are both fully stocked with ammo. Meaning there is a seemingly infinite, but instead, large supply of ammunition.
List of appearances