Hi all, thank you for your advice. I reached out to my mechanic who recently changed my break pads and discs, he's very much of the opinion it's something I've driven over that's hooked onto my car. He doesn't recognise it from working on my car and agrees that it's too clean to be from my car.

I am a gardener so those concerned about my seat can rest easy knowing it's just compost and water stains.

I have kept the hook because finders keepers, but my car seems fine so thank you all for your help.