On the Fremont Street level there is a bottle of Axe on a ledge. Shoot the bottle and the roof on Fremont Street changes to an animation of a girl in a bikini with some Axe materials in neon. A movie opens up showing some outtakes and pretty funny Rainbow situations/early animations.

After destroying the opposition in campaign (single or multi-player) don’t forget to pick-up each new weapon you come across. While you don’t have to keep a weapon and finish a map to unlock it in your customization menu, you do need to hold onto it briefly or your profile will not save it.
Another advantage is once you “claim” the weapon, you can die or restart the mission entirely and use it in the stage you got it from. This way, all the enemies you drop pretty much restock that weapon’s ammo and you literally will be carrying a portable ammo dump throughout that stage.

Note: Hil-IGN says you can find the riot shield somewhere in Act 2 of the single player game, but SNG-IGN playing on co-op noticed it did not appear.