Returning cast member? Since Morgan was killed off BC has only been back once. He came back to torment Sonny when he was in another mental health spiral. I wonder if they'll do the same thing this time but maybe this time he guides him to get help with his meds? And considering Ava messed with Morgan's meds and is now essentially being complicit in Sonny's meds being tampered with, that could also be tied together. If it is BC coming back, I don't think it would be for long.
Or maybe he haunts Ava into telling Sonny. Lol