To get the Argus Mercenary costume in the Machi Akko level, when you reach a hallway after falling out of a vent, there will be three room that require a lockpick to enter. In the third room will be a closet that cannot be opened. Continue to the bar, then go on stage and use the microphone. Sam will say something stupid. After he is done, turn around and look behind to see a keycard. This keycard will open the closet which contains the costume.