Find the movable crate in the wall next to the stack of smaller crates and pull it once.
Go around to the left side of it and pull it again.
Go back around the other side
and use the CROWBAR to break open the wooden door. (Position Lara at the center of the
door, facing it, and press Action. Select the crowbar and press Action again to use it.)
Enter the room behind and pull the large wooden crate on the floor once to get the MP5 clips underneath. Climb onto the row of smaller boxes against the right wall and pull the corner crate out twice. Go around to the other side of it and push it once into the corner.
Pick up more MP5 clips (circled) from where the crate was. Pull the crate in the corner (just beyond where the second set of clips was) once.
Then go around to the left side, find the fourth crate, which is flush with the wooden wall, and push it into the corner onto the spot where the third crate had been. (This screenshot was taken after the crate was moved.) This is secret #2. (The large medipak is circled.)