In Chapter 3 when you start on the roof of the multi-storey car park you can jump on the barrels and then onto the roof near the exit ramp. If you shoot the peculiar looking metal wall you'll see it collapse. Then go through the opening, drop through a nearby vent in the floor and you'll end up in a small room. You'll find a sniper rifle and some ammo and a radio. "Use" the radio for some laughs.