There's no reason in the world that they will look askance at your "open jaw" trip.
There are no reports I'm aware of, of AA (or any carrier) peeing their pants over a single hidden city ticket.
In the wildly unlikely event they did make an issue of it, I'd just tell them I went into town during my 6-hour layover and lost track of time, and felt too embarrassed to go back to the airport so you went from B-C on your own. (Your plans obviously contemplated going from C-B on your own later, right?)