My ‘take’ from the beginning is 1. He was upstairs with headphones and didn’t hear a thing
2...she was brutally killed but opted for the basement to protect her son
Her son was clueless
I also was thinking the 13yo son might’ve been wearing headphones, probably playing videogames or video-chatting with friends.
But, I don’t think this woman was very concerned about her 13yo son. She made poor choices and engaged in reckless behavior. She left him home alone to go out late at night. He is lucky the killer didn’t attack him, too.