Citizen Alert
From Halopedia, the Halo wiki
On Metropolis (Level), near the area where the player fights a trio of Wraiths (the Marine Command Center), there are public bulletin panels scattered about with public messages to civilians. There are two variations of the sign, each with a different message concerning the battle with the Covenant. These signs are similar to the 4 that are visible in the Halo 2 E3 2003 Trailer, when Master Chief mans the turret and after one of the marines on the radio says "Hostiles right". It's unknown if the messages on the boards are the same as in the final game.
