After you beat the game, use the unlock all levels code and start a new game and select the Bygone Marshes (or just start a new game without the code and get to that level).

Turn around at the very start and there is a long area that you can drive through. So, drive through the pillars until you get to the last two sets of pillars. On one of them (facing north-east [use your compass]) there is vandalism that says, "Jack Archer Lives!!"

If you shoot that particular sign, a health pack will drop, followed by protoculture, then a Holden Mark-2 Shotgun. There's one more item that comes out, but you have to keep shooting the sign until it does. If you do, you'll get a soccer ball. You can shoot it with the shotgun to make it bounce all over and test out the Havok physics engine.