This is going to sound trite, but you have a serious leak. Clearly something to do with the tank, and not the fuel system as I assume the car was shut off while you were filling it. This is not one I've heard of, I suspect not a common issue, you are going to have to look, or have someone else look. Should be easy to find, although being easy to find may not mean it is cheap to fix. Anyway, I don't think you should drive the car, it sounds like a fire is a very real possibility. By the way a 2003 is a B6, this is the B5 forum, for future reference you will get better answers there.