From Halopedia, the Halo wiki
The Lost City is a visual look-development level that was developed during the production of Halo 3. The level was based around a Forerunner city and was described as being so immense that ""it could be a game in itself".
The Medical Troop Warthog is a Warthog type included in the files of Halo 3 but is inaccessible during gameplay. It's appearance is that of a Troop Warthog, but with two benches being replaced with stretchers.