Well, its a yellow car with a black engine bay, meaning that the enigne bay was probably painted black... To cover up rust
Chances are that the rust went through and some previous owwner painted over it. Thats a very common place for rust as the battery acid mixes with water and rusts away the wall and the hole at the bottom.
I had rust in my car, same places, almost exactly the same shape, etc.
I would strip that paint off and see what is really going on.
-Tony