In any stealth mission where you sneak around and don't want anyone to
see you, if a guard goes "?!!!", its a good idea to take cover quickly
or kill him silently or he'll scream "ALERT!" and possibly turn on the
alarm which you don't want to happen. Also, in any stealth mission, when
you kill someone hide them behind crates, vehicles, in small closets,
etc. or where nobody else you think  well go, a lot of times if you
don't this a wondering guard well run into a unconscious body and raise
the alarm and you'll be in trouble.