So here's what you could potentially expect to see. This is also why it's hard to stop rust by patching sometimes.
My car had some collision damage from long ago that was covered up with filler. The damaged was by the door, this is lower rear. Long story short I decided to just replace the quarter. This is what I found, from the outside it looks similar to what you have going on. There was no sign of rust from the trunk drop off either.
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