The hidden sign is an easter egg in Grand Theft Auto III and Grand Theft Auto: Liberty City Stories left by the designers for the player to find. It is a large message reading: "You weren't supposed to be able to get here, you know" in GTA III and "Hello Again!" in the PSP version of GTA Liberty City Stories. In the PS2 version, the sign says "You just can't get enough of this alley, can you?"
On Staunton Island, go to the left of the main entrance of the car park that was used in the Kingdom Come mission. Go up the stairway you find up the alleyway with a car and park the car by the wall to the right. Jump on the car, then on the wall and you will be on a flat roof. Jump off the wall to the area where you see all the fire escape stairways and look around, you will then see a message on one of the walls.
|Direction to the "hidden sign" on YouTube|