Found in the West Office of the Guardian Citadel's Grand Hallway, north of the Armory. Guarded by Sister Wrath, his spike-wielding secretary - after you dispatch her and move past her location, you get the following text:
From inside the west office you hear a voice call out, "I thought I told you I would see no one today, Sister Wrath."
Ignore this warning and enter the doorway to the office, and you get the following text... and a grenade!
A live grenade bounces into the doorway. "Chew on this, Infidel slime!"
When you actually encounter him, you get the following text:
The Guardian facing you sighs. "Why does it always happen in my shift?" He raises a strange weapon and shoots.