Ford Focus sills

Maybe coincidence but I've had three of these in over the last couple of weeks. All over 10 years old and all with a soft patch in the outer sill (and I mean easily push you're finger through soft). But the odd thing is all three had gone in exactly the same place. Rear of the nearside sill whilst the rest of that sill and all of the offside one were solid as a rock. No external paint damage, in fact the paint was pretty much all there was, so gone through from the inside.

And, before anyone asks, proscribed areas for suspension component and seat belt mounting!