Boothy, some pictures of the offending rusting area might help to direct people where to inspect on there own cars, they might be located elsewhere in the threads you talk about maybe even a guide of where needs to be inspected to pick up on this issue early doors potentially before exterior paint bubbles.
Snoopy, very interesting about the sponge on the mk6, that area on my mk5 had a massive collection of leave and other detritus, I tracked it back to the ventilation vents at the base of the windscreen, where I found another large collection of leaves and grime, to clean it up properly you need either really small hands or to remove the wiper and plastic strip at the base of the windscreen.