I also lived on the first floor alone next to the kitchen while my other three roommates lived upstairs in my apartment, but I was super close with my roommates just as these girls seem to be. we always had daily check-in‘s after a night out and run straight into each others room whoever woke up first. To me that was normal in my house. So I would have discovered it that’s just me so I’m just basing it off of how I would have discovered it. And majority probably would have too. It’s just not really clicking to me the storyline. They seem super close like sisters/ bffs. So a real quiet house or not seeing each other would definitely be an immediate red flag for me.