After I finished it, I went back and tried something I’d heard a rumor about. The point of the game is to complete your mission – get a package, assassinate a target, rescue a hostage – without anyone suspecting you. You can shoot out lights and hide in the dark, change clothes and pretend to be the pizza guy or the chauffeur, chloroform guards to knock them out, etc. You get rated on how little commotion you make – novice, professional, expert, silent assassin if you ace the mission.
However, there are a few other hidden ratings. For example, if you manage to kill everyone, without anyone seeing you, you get rated Ghost. I tried it out, and sure enough, it works! This screenshot is from the Graveyard Shift mission:
For more information, check out this forum thread.