Not sure if it fits your criteria but I once read a book where a teen foster child has surgery and then wakes up midway through and the doctors are stunned because he has mechanical parts.

After some running away from government agents and a love interest who gets killed he faces off with this particular agent. Gets the best of this agent and then cuts his own body open so the agent can see the part. He then says something like "you want to know what I am so badly, well take one last look because you will never know and it will drive you mad" Wound heals (he also healed quickly) and buggers off never to be seen again. The book then ends.

We didn't kill your girlfriend, Protagonist why did you have to do that to us.

It's a mystery chase novel. The chase was OK but chases aren't why I read a book and the mystery is never revealed.

I now realise that I can't remember the title and that's going to bug me for the rest of the day.