On the way to Room 4 it drops a key every time you walk by.
You can leave, walk by, and enter the room over and over and collect the special goodies.
I was able to make about 30k in a couple hours. (picking up and selling the goods)
Maybe it is on purpose, but I don't assume it should be that easy to gain a fortune with so little work. :-)
(The key should only drop once per person per game day, in my opinion.)