When a patrolling guard sees you, run into a room where the screen goes black and tells you where you are. The game will not go into evasion mode, alert mode, or caution mode.
With a silenced USP/SOCOM, you can shoot at pipes. They are quite common in the game, and, when hit, shoot out steam. This will paralyze or distract an enemy, depending on their proximity to the pipe. Fire extinguishers have the same effect. With a silenced weapon, you can shoot a wall near a guard to make them turn and look at the wall for a few seconds. If you evade a guard's detection in hanging mode and they walk past, press ACTION. Your character will flip over the railing and knock the guard out with the impact. Alternately, you can fall on a guard from hanging mode. If you don't have ammo, unequip your weapon, run at a guard, and when close, press CROUCH. Your character will either do a somersault or a cartwheel and the impact will knock the guard out. if all else fails, toss a chaff grenade, run and pop off all the guards before the radios work again.