Like the previous Gears of War titles before it, Gears of War: Judgment hides a host of secret locations that can only be accessed by only the most keen-eyed of players. These hidden rooms can be found by discovering secret switches located in some of the game's many maps in multiplayer.
To access these areas, just hit X when you find one of the hidden switches and you'll be able to unlock secret passageways and corridors to pass through.
These secret locations exist in the multiplayer maps, so it's in every player's interest to find out where these areas are and either camp those spots for the items they provide or use them tactically.
The first video contains all of the secret rooms on Library.
Likewise, if you're playing the game on Overrun mode and take on the role of a Ticker, you can run through a variety of holes on the map that are inaccessible to any other player. These little holes will allow you to stage ambushes on the enemy team as a Ticker. Be sure to mute the sound on this one.