There are plausible reasons to believe the body was covered by a blanket, hiding any blood loss that would have been absorbed by mattress/sheets.
That being said, I do not understand the Gen Z of it all. They had to call friends to come over even when they found a roommate unresponsive. If this was NOT a murder investigation and the unresponsive roommate was actually passed out from alcohol poisoning, imagine explaining to their parents that they really wanted a friend to come over first to call because they don't like talking on the phone or "they're scared" of calling 911. Or whatever their reason may have been.
Further, I do NOT understand how this same roommate needed to call friends to come check on a person in her house instead of going to the rest of the rooms in the house for assistance. Police have confirmed several times now the police officers found the other bodies.
Honestly, when you think about it, I'm not shocked they spent a few days staring at the roommates like they knew something. They've cleared them now but good grief they didn't behave in a way you'd expect someone to in an emergency situation.