That rotten egg smell is pretty normal in turbo'd cars- I get it all the time, and has to do with the sulfer in the gas. It is NOT a sign that the cat is necessarily bad, but its working. You might try a different brand of gas if this is truly occurring frequently-- otherwise ignore it if its not causing other problems. If your state has emission standards, and actually test the emissions of the car, then you can tell if your emissions are off.

The thing that will most commonly give off a true rotten egg smell is the catalytic converter. It is a little worrisome that you smell it with the internal recirculation on. If it is not quite a rotten egg smell, a little sweeter, it could be a leak in your heater core. Any wetness on the floor? Can you smell the odor at the exhaust when standing outside at the rear? If it is the cat, you should smell it there. Since you smell it inside with the recirc on, then if it is the cat you may also have an exhaust leak that is letting the fumes get in to the fresh air vent.