What is the metal I have in my wall?
What makes you reasonably certain it's not a protective plate? I ask because this is almost by definition what a protective plate does.

This is likely protecting an electrical cable. When you run wires to outlets or switches, it's good practice (and required by code in many places) that you place protective plates on the wooden studs where electrical cable has been ran through it. This is an important safety measure, to keep you from drilling or nailing into a live electrical cable.

for example: