For myths in GTA III, see GTA Myths Wiki.
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 werent supposed to be able to get here you know" (with the missing apostrophe) 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 inside the car park that was used in Kingdom Come mission (if you don't remember this mission the car park is located in Bedford Point). Get a car and take it around to a underground car park right nearby, a heavy and big car like the Trashmaster will be useful. You should see some stairs ahead of you. Park the car at the top of stair and get out. Jump onto the car and then onto the roof. Walk across the roof and jump into the nearby alley full of trees. After landing in the alley, walk towards the other end of the alley to find the sign on a wall.