I had a guy in who I discovered was NOT an electrician. He rewired my outlets (among other things). My upstairs outlets are wired on circuit 5, until the second to last outlet–and until that outlet they all work. That one does not have the trim on it. It is fed by #5, but then on the other side is fed by #17. #17 feeds an outlet that reads not grounded, and then feeds this one.
The fact you don’t know and that I question the aspects of your description…call someone qualified to make judgement would be the best course of action.