From Halopedia, the Halo wiki
Lost Platoon is a Firefight map in Halo 3: ODST. It is one of the larger and wider of the Firefight maps, as it contains a Warthog at the map's centre structure. Unlike most of the other maps that are too small or that just let the Covenant drop off Wraiths, this map will also let the Covenant drop Choppers.
Based off a section of the Campaign level Uplift Reserve, Lost Platoon is set in the Uplift Nature Reserve, a park located next to the metropolis of New Mombasa, which became a battleground during the Battle of Mombasa. By the time the map is set, the New Mombasa Orbital Elevator is still standing in the background. The map consists of a series of buildings connected by walkways in the center of a scrub land-like environment. The name of the map is a reference to the Second Platoon, which is holding position in the area during the campaign level.