After you rescue the second hostage, you have to go into a giant freezer. Take out the guards on the cat-walk then get into the crouching position. Turn on your thermal imaging goggles. There will be guards, but they will be blacked out by mist and the carcasses of dead cows. However, with the goggles you can see their feet. Shooting them in the feet or legs enough times will kill them, saving you and your men from exposure.