If we look at only the information available to the surviving roommate, she knows only that:
1) Kaylee was with her dog.
2) A man was crying in X's room saying something along the lines of "I will help you"
3) A man with bushy eyebrows walks past her room and out the door wearing some type of mask.
She knows nothing, as far as we know, of stabs, blood, deaths, any violent crime, cell towers, DNA, cameras, Elantras, or a knife sheath. As far as she would know at that time, the man in the mask could have been the food delivery guy.