Rust in that unlikely area indicates possibly the car was hit there in the past and the repair was pulled out. Areas that are hit and pulled are more prone to rust because of the way the metal is worked.

What is the molded black piece to the left in the photo, is is carpet ? I don't think it is the gas tank as it just looks too far forward and in the way of where the spare tires goes. Just curious the pic is so close up I am losing perspective.

An old man once told me that an old man once told him that rust never sleeps.

-Don B

It's the latch panel, with either very oxidized paint or undercoating on it.

Thanks Dr Jekyll

-Don B