Best option it was just an area where moisture accumulated. If not you really want to have a look around that car. That is really heavy scaly rust where the metal has returned to primarily just iron oxide. When it looks like this usually you will find it separated in areas throughout the metals thickness. It remains partially intact but very weak.

You are right in that the undercoating can hide things really well until they structurally cannot support it anymore. If I find rust like this and I live in a rust belt. I am going to carefully probe important areas. Hopefully this is just a localized issue. Yet for safeties sake you have to make sure.

As an example my neighbor had a ford station wagon. He dropped over with his car to use my shrinking/stretcher metal shaping equipment to manufacture a patch for his rear roof just above the rear door of the wagon.

He asked me how I would repair one of his rear shock towers. A quick check indicated he had no holes in the floor but it was totally compromised right from the back to the front. I could easily poke an awl through almost anywhere. So I told him since I was not involved in the metal repair. Scrap the car.

I had also put it up in the air and checked it before giving this opinion. He too is dead now as the years passed. Well before his demise. A car hit his with his wife driving. The car folded up really bad and killed her.

I think the amount of corrosion in that car was a major factor. The odd thing with that car is most the sheet metal topside was in decent looking condition. The roof metal failure just above the rear door I thought strange initially.

Possibly Ford used a lot of poor recycled metal in their floors and chassis structural components. I think if I remember it was a little ford escort wagon. I also saw a Vega station wagon we owned ourselves lose its floor in the same manner in those days.

Where am I going? Mercedes uses thick metal rocker panels for unibody support. The area you have an issue in should undergo a strong repair. If the rust is locallised. Structurally the area is signifigant.