Afghanistan - 3 briefcase
Iraq - 3 briefcase
Aircraft Carrier- 3 briefcase
China - 3 briefcase
Miami - 2 briefcase
You get a cash reward for picking up the briefcases and they can only be collected once. It disappears in future playthroughs once its collected. You can check 'Objective' in the pause menu to see how many you have collected for the level. Collection is accumulative, ie, you don't have to collect all in 1 playthrough.
#1: Behind the first M-11 missile is a small uphill path, follow it all the way to the end.
#2: After encountering the 1st heavy armor guard in the mission (a cutscene plays showing him walking out of a door), kill him and all the guards and search the platform area. It's up past a short set of stairs at the back of the platform.
#3: In the area just outside of Brian Hick's cell, go all the way to the back to get this one.
#1: Right before ascending the ladder to take out Ali Youseff's Lieutenant's Helicopter, look around at the bottom of the ladder to find this.
#2: When you're asked to call in the air-strike on the bunker, run to the tent nearest to the huge wall dividing the area (there are some sentry towers near the wall), it's on the side of the tent facing the wall.
#3: In the final area of the mission before meeting Ali Youseff, there are some metal platforms with guard on it. Kill them all, climb up the small steps to the 2nd level of the platform, and then the ladder to the 3rd level to find this.
#1: On the main deck of the ship after you land, run towards behind control tower and then to the right to find this on a platform on the side of the ship.
#2: In one of the hangars, you have to operate one of the planes to blow through a door (a heavy armored guard appears once the door is blown). Search around the wall closest to the door. It's behind some missiles.
#3: In the final hangar, after activating the missiles to sink the ship, look around at the back right of the hangar to find a basketball court. It's behind the basketball hoop closest to the hangar wall.
#1: Once you arrive at the dam, head up the stairs towards the first control panel. Look across the dam to see a similiar structure directly opposite. The briefcase is there.
#2: Once you arrive at the village, co-op snipe the 2 guards and take the right path. At the first house you can enter on the path, head to the 2nd floor to get this.
#3: After the only Back-to-Back sequence in China, do not enter the factory. Search outside near the back outside the factory (it's behind a dump) to find this.
#1: After the gates to the luggage area are opened, you have to run to the 2nd floor to operate a control panel. There's an office right before the control panel, search around the back of the office to find this.
#2: On the 3rd floor of SSC tower where you have to jump onto the lift, ignore the lift and keep running forward to the back of the floor to find this. It's directly opposite and across the staircase that you came up on.