Well, just look at what happened to the woman in the story who opened the door when someone knocked.
I forget which freeper said “Nothing good ever comes from a knock on the door.” Very true!!
Everyone in our family says “Who is it?” when the door is knocked on. If we don’t know them, it doesn’t get opened. We live in a low crime suburb - but that’s the rules. Includes police officers, etc. (I have cracked the door for a cop with the chain on after confirming the squad car in the street). I have an additional rule when going to the door.