It all depends how close you are and what type of friends you are with your roommates. My junior year I lived in a 4 bedroom house with my best friend and two other random people. We never knew what the other two were up to at all because we were purely roommates and not friends at all. We only all sat down for pre planned roommate meetings every few months when someone got mad about a mess or noise another person was making or something.
I guarantee one or both of them was gone for days at a time before and we had no idea. Just assumed we were on different schedules or something because if a bedroom door is closed they could or could not be in there. Apartments are different then dorm rooms where the other person's absence is more noticeable (no doors to hide behind).
Sorry...I just don't see her not communicating with someone throughout her "travels" during the night. She was very social, had many friends, was in various clubs and activities in school, and even attended parties with friends up until she left for the mall area. If she wasn't close with her roommate(s), she was close with other people that I assume would have been concerned about her not showing up at all the next day! I am not saying this communication never occurred. Actually, I bet it did occur (based on her social activities and appearance of normally being intelligent and responsible) we just don't know about it!