I've only found 3/4 Keys. Gold Key, Jet Black, and Hefty Steel. No Brass. Does it have to do with the idol? I've had no luck with figure out what that works with either. Found the Brass Key. It was just tiny enough that it's a PITA to see, much like the Jet Black Key. Still no idea what to do with the idol though.
The Green Balloon being a red herring is... annoying.