On mission 1 (where you attack the guard and escape
from the cell), in the courtyard, go to the room where there are
two guards warming their hands. Note: If you have triggered the
alarm this will not work. Sneak up behind the one who is holding
an MP40 and backstab him. The other guard should not notice. If
you get too close he will begin to turn around. If this happens,
it will not work. It will also not work if it takes more than
one stab to kill him. Note: This was done under the "Don't
Hurt Me" difficulty setting.