These cars too suffer from 90's pvc plastic undercarriage dip. Many european car makers used same stuff and their cars rust almost as much as these do. The thing is that surface looks great way too long but under the paint and pvc film rust does it's job peacefully. In the end you have nothing but rust particles between inner and outer paint layers. Top of that, to my eyes these cars first owners did not care at all of any rust proofing. There is like nothing else but thin layer of paint or dip inside of sillpanel. Wich is odd as most of them may have been knowing how easily and badly x40 etc rusted out.
I've have pressed medium size chisel end screwdriver against peugeot 406(mk1) from the late 90's sill panel. It looked fine. Body color was on quite nicely etc. Screwdriver went thru sill panel like nothing and I could pull it all across the panel with ease. Like hot knife to butter.

This spring I welded the same spot as topic starter has. Both sides. Not as nearly as bad has he had. Last weekend I lifted old dried rustproofing layer behind front wheels at the front end of sill panel. a layer of 10x10cm dropped to floor from both sides. There is no welding job to do. I'll have to lift the rest of the dried rustproofing out and eat away the rust before I have to do anymore of welding.

Yeah old post but rust doesn't just vanish.