In the first level after the second checkout take a right when you have walked through the door. Shoot the damaged door that you find there and then walk through the corridors. This will take you to a secret theater that shows previews of The Wheelman and Blacksite: Area 51. You will also find in the theatre a shotgun, golden pistol and a paper crane.
Use Tequila Bomb for Precision Aim and shoot someone to create an extra 20-30 rounds of ammo when you run out. Obviously to do this you'll need enough Tequila Bomb for the Precision Aim so try to keep enough in reserve for emergency ammo.