On the mission 'Curtains Down' there is a basement on the west side of the building with a hidden room. Kill the three rats which are in the room and a key card called 'Ratclub Keycard' will appear on one of the tables. Collect the card and go up the stairs beside the targets dressing room to the second floor and use it to enter the first door on the left. Inside is a storge room with lots of rats having a party and enjoying themselves.

When you save during a mission a message box will appear on screen telling you that the game is saving. Press start before the 'Save Complete' message appears and it will not count it towards your limit. No harm will come to the saved game.