On the opera house mission, in the basement at
the back part of the stage, there is a room that
leads to staircases to bathrooms on the first
floor. After heading down the stairs there is a
room you enter, kill the three rats on the
ground and check the bench at the end of the
room. There is a keycard to the rat club. Head
to the second floor where you get the lightroom
keycard. There is a door that can't be opened.
Use the rat club keycard, and in there are rats
gambling and fighting.