I don't use magnets, or stud finders set to 'metal', ever. I use drill bits. If there's protection plates, the drill bit will tell you that.
I think you're finding screw heads. Disclaimer : I live in EMT land, so there's generally no concern beyond hitting a PVC pipe or low voltage comm.
If you have to push really hard, don't; relocate. Patching and painting my be required after all is said and done. That's why it sucks trying to hang something that won't cover your booboos. And why 'new construction' people won't touch rehab jobs that begin and end with 'unforeseen circumstances'. This is your can of worms; open it up:
You're going to have to do some recon... with a drill bit. Bite the bullet and start making holes. And use your intuition: It's an outside wall so it's unlikely there's any plumbing. And way up there would be a strange place for an electrical run, but anything's possible. - Henry Jones says: "I don't know. WE'LL FIND OUT!"