In Hoodlum Headquarters, you know the part where you hold a ball thing while you walk across a bridge with red stuff on either side of it? And then a Hoodlum accidentally shoots himself in the head? Well, after he shoots himself, keep walking until you get to the other side of the bridge. Then turn around and go into first-person mode and look into that crack in the wall that the targets are going into (or coming out of, I can't remember which...). There are at least three yellow gems in there, but I have no idea how to get them. They're in the Gamecube version, but I don't think they're in the others, as nobody but me seems to have seen them.