This one was in the RF wheel well, against the passenger footwell.
...and once again, I forgot to take pics. Dang it. Well, if you look past the ground-down welds, you should be able to see the roughly Africa-shaped patch here. These repairs often end up being roughly twice as large as the original rust hole, so you can use that rule of thumb to figure how bad it was when we found it. (BTW, that line of three holes on the side belong to the mudflap fasteners.)
Oddly, the other side of this car shows absolutely no problem in this area. So maybe this wheel just happened to kick up a random piece of road debris at some point, enough to compromise the undercoating and start rusting? Who knows.
At least it’s patched now. It would have been all too easy for all kinds of water, salt, and Detroitus to get flung into there. Next up, the driveshaft-arch/control-arm-mount/frame area...