E E E
From Halopedia, the Halo wiki
The sign can be found in the area with the three Wraiths, just before the building with pinned down Marines. The sign is located on the road heading left when entering the street for the first time. In order to view both versions of the sign and unlock the achievement, the player must view the sign in the remastered graphics, as this causes the sign to flash back and forth between the two versions; in classic mode, the sign stays locked on one version.
The name of the achievement is 3 'E's; this is a reference to the Electronic Entertainment Expo or 'E3'. The Citizen Alert made its first and only other appearance in the Halo 2 E3 demo, shown at E3 2003.