Lucas wants to go to the secret hangar. Get up and wake up Markus. Get out through the window.
You have to get past the watchtower and the patrols. Follow the path that's on your map.
Pass the first watchtower and wait until the patrol doesn't look at your direction. Continue your way. Lucas will notice a soldier. Now go back to the car and hide behind it. The soldier will pass you by. Now you can continue your way.
Lucas will notice another soldier. Go back, find a rock (the place is marked on the map). Lucas will throw the rock and the soldier will turn around. You can continue your way. Now you're playing as Lucas. Follow the path that's on you map. Then you will automatically switch to Markus. Go to the place where Lucas is waiting. Together again, continue your way. You will see another soldier.
Now Markus will say that he will distract the soldier. Go to the place which is marked on the map. Kick the barrel. Now you're playing as Lucas. Climb the pole.
Watch out for the lights. Go down and run to the hangar. Open the doors and go inside. Now to the lift and... you will see nothing :-).