Two workers are encountered on the Citadel: Expose Saren mission right after the fight in Chora's Den and before meeting Fist. Shepard can convince them to leave without fighting, or can just shoot them outright.
Warehouse Workers are among the weakest enemies in the entire game. They lack the shields that the Bartender and the Thugs have and they are only equipped with pistols. They will try to take cover but because of the lack of defenses, they aren't that hard to defeat.