The Ghost Wall has three locations, one is located in Vice Point, in an unreachable region of the map and the other beside an alleyway in Ocean Beach. The wall has unusual attributes, as it becomes invisible if viewed from the east. But it can be seen when viewed from the west. Another wall can be found in a garage, in Little Havana.
An explanation for this effect isn't disclosed during the game, however, and it is most likely a mistake made by a developer. In-game textures can be applied to either one or both sides of a 3D surface. In this instance, it seems an artist has neglected to apply the texture to both sides of the object, the game, therefore, renders one side with a texture and the other side as blank space.