Throughout most levels, you may come across a guard named Pierre who is writing a message to his mother (or in one of them, a girl writing to a friend about how all this guy Pierre can talk about is fish and cheese). As a running gag, it's pretty effective. However, since Pierre is, after all, a guard, he is at odds with the thieving crew, and can become antagonistic to the crew if they catch his attention.
This leads to the logical conclusion that one may, if carrying a weapon, kill Pierre.
He appears in new levels thanks to the thieves' actions, which cause him to be fired from his current job, so it's possible he might disappear or be replaced if killed. Then again, guards seem to be able to revive their own through some magic grisly process I don't understand (or care to).
So, in the campaign, if you kill Pierre, does he show up in later levels to write more messages?