Checkpoint B. Immediately after entering the building and climbing up onto the 
  vent, you will find the bag (required for level completion).
  Checkpoint B. Drop down, and survive the guard ambush. Run up the red stairs, 
  and go through the first red door. Turn left, and run to the end. Climb on the 
  storage shelving to find a bag on top.
  Checkpoint C. After you are ambushed and run across the rooftops, you will 
  reach an area with two guards. Fight them, then go to the blue stairs and 
  walkways. Go underneath them to find a bag hidden between the vents.