You will reach a hallway after you fall out of a vent. There are three rooms in this hallway that require the lockpick to enter. In the third room will be a closet, just like the other ones; however this closet cannot be opened yet. Proceed to the bar, then go on stage and use the microphone. Sam will say something dumb. After he is done, turn around and look behind. A keycard should be somewhere. This keycard opens the previously inaccessible closet. Make your way back to this room and use the keycard to open the closet; the costume is inside the closet.