"Because I exorcised the evil bastard possessing him. Turned out there were a few other cases this spirit had caused spanning fifty years. Generally, though? Look for behavior that's a bit outside the norm of the person you know. Violence, aggression. Stuff that comes on them suddenly. Who knows, maybe some pots go flying, glass breaks. Especially if the bastard likes a bit of showmanship."

He figures it can't hurt to tell Negan. Who knows what Mathias might throw at them next?