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 the briefcase.

2. When you are 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). The briefcase is 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 guards on it. Kill them all, climb up the small steps to 
the second level of the platform, and then the ladder to the third level to find 
the briefcase.