Being a bit of an anorak, when I'm in scrappies, I pay attention to how Golf's rust out so that I can protect mine.
I have noticed that some even quite late mark twos are badly rotten in the rear quarter below the fuel filler. I have never seen a posting about this so , having just lost my virginity, I thought I would chuck in my tuppence worth.
This rust comes from the inside out and I am pretty sure that it is caused by the drain in the filler recess becoming blocked with road dirt etc. The water ponds in the recess and gets through the gasket at the filler neck and drips down inside the quarter panel. If after rain or when you wash your car you open the filler flap and find a puddle then make sure you poke about with a bit of coat hanger to clear the blockage.
You won't be able to stop yourself from checking tomorrow will you?.