You will get money for picking up a briefcase. Each briefcase can only be collected once and will disappear in future playthroughs. Pause game play and select the "Objective" option to check many briefcases you have collected for the current level. Note: You do not have to collect all briefcase in your first playthrough. You can find missed briefcases during subsequent playthroughs.

Afghanistan:
Look behind the first M-11 missile you find to find a small path going uphill nearby. Follow it to reach the briefcase.

Kill the first heavy armor guard you encounter. After the intermission sequence, search the nearby platform area. The briefcase is on top of a short set of stairs at the back end of the platform.

In the area next to Brian Hick's cell.

Iraq:
Just before climbing up the ladder to take out Ali Youseff's helicopter, get the briefcase at the bottom of the ladder.

Call in the air-strike on a nearby bunker, then search for a large tent near the dividing wall. The briefcase is against the wall near the tent.

Just before meeting Ali Youseff you will see metal platforms with armed guards. Kill all of the guards then climb up the steps to the platform's second level. Climb another ladder to the third level to find the briefcase.

Aircraft Carrier:
Land on the ship's main deck and run to the right of the control tower to find the briefcase.
When you enter the hangar to use one of the parked planes to blow through a door, look behind some missiles to find the briefcase.

After setting the missiles to blow up the ship in the final hangar, search until you find a makeshift basketball court. The briefcase is behind the far basket.

China:
Go to the dam, then move towards the first control tower. Across the dam is another control tower with the briefcase.

Go to the village and co-op snipe some enemies. Follow the rightmost path. Enter the first house you can explore and search the second floor for the briefcase.

Go to the factory, but do not enter. Search behind the factory to find the briefcase.

Miami:
Open the gates to the luggage area then go to the control room. Search the smaller office near the control room to find the briefcase.

Go to the third floor of the SSC Tower. Instead of operating the lift, run to the far end of the third floor to find the briefcase.