Mansion was a planned multiplayer map in Call of Duty 4: Modern Warfare. All that is known to prove its existence is a file called "compass_map_mp_mansion.iwi" present in the game's source files. In addition, its name is known because it is referenced in the game's mod tools in the file "mpui.str" in the raw/English/localizedstrings directory.
The map is focused on a single building, a mansion. The map itself is extremely small. Aside from the mansion and an unknown structure outside of it, there is nothing else in the map.