Suburbs 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_suburbs.iwi" present in the game's source files.
The map seems to be set in a town with oddly-shaped buildings, and with cars scattered about.
A set of two circles - a green one and a larger blue one - were placed in the map and were most likely used to determine how big the map should be in relation to the range of weapons. The two circles can be found in the game's mod tools; they are a model to assist the player determining the size of their map. The green circle represents the range of submachine guns, while the blue circle represents the range of assault rifles.