As I see it, all she would know was that a germophobe friend of one of her roommates came over, and then let himself out in the middle of the night.
The noises about that time may have been disturbing, but didn’t constitute a call to action, the way a scream would have. His body language may have alerted her to something ‘off,’ but nothing that she thought warranted calling 911.
Sadly, it’s very easy for me to think that her thoughts ran along the lines of: “In the morning, I’ll ask what was going on.” And that seems reasonable to me.
MOO