2004 Toyota Prius body rust / corrosion (rocker panels, front fenders)

Mud flaps are likely part of it. They might as well be called mud trappers.

The more inaccessible the manufacturers make a zone, especially down low, the better the odds it's gonna accumulate gunk and subsequently rust.

On our 3rd gen I've found both front and rear wheel well liners have at least one fastener that pretty much needs to be destroyed to remove. That plastic rocker panel too confounded me so far: just a mess of tricky, fragile fasteners.