Start the mission and proceed all the way until you get to the first building. After the first floor is clear, make your way upstairs to breach the doors. Breach all doors on that level until you get to the last door to breach on that floor. Once your squad has announced all clear, make your way into the bathroom, which should be to the right of the room as soon as you breach it. Look into the bathtub to find a blow-up doll with colorful stars on its chest and a circular mouth.