Okay but jokes aside, when it comes to demon possession, "odd or abnormal behaviour" means abnormal for this specific individual. Everyone knows about the stereotypical symptoms like talking backwards or walking on the ceiling, and jokes about how you'd know that someone who's incredibly weird by nature would probably start acting more normal if they got possessed, but apparently it's not common knowledge that this counts as abnormal behaviour. Like obviously someone who's been strange as hell their entire life hasn't been possessed this entire time, but if you know someone who is odd, and has always been odd, you might want to start asking questions if they suddenly start doing things you've never seen them do before, like sitting on chairs the normal way, or using kitchen utensils for their intended purpose.