Rat Club Access!
Load up the mission "Curtains Down." Get yourself to the basement of the mission, in the building where the target is. There is a place in the basement where there are three rats. You'll know these are the right three rats and the right basement because a worker comes in and tries to kill them with a nail gun. You kill all three rats yourself and then a Ratclub Keycard magically will pop up on one of the tables in the room. Take the stairs near the dressing room of your target. Go to the second floor and use the keycard on the first door to the left. You should find yourself in a storage room. There will be a cell that separates you from a portion of the room. You'll know this is the right spot because on the other side of the cell will be a little rat party!