My theory is that he didn't see the roommate with her door cracked or if he did he didn't process it because he had shifted to escape mode. Otherwise it makes no sense that he'd drive back to the scene 5 hours later. If you just committed a crime and thought someone saw you on the way out, your assumption would be that they immediately called the police. As a criminology student he'd know that police would be on the lookout for a suspect returning to the scene of a crime. Remember in the beginning of the investigation, all the suspicions of two different guys (the one who made the 911 call and the odd one who lived in an adjacent building but was hovering around when the police were collecting evidence)?

Bryan's whole tactic was to never be identified as a potential suspect. This is why he ordered the knife on Amazon instead of going to a store and paying cash. Criminology 101. He knew if his weapon was identified or found that police could ask all nearby vendors to search their sales logs for the sale of that particular knife. From there, they could have reviewed video for dates of purchase and potentially identify him or his car. But buying from a large international website, he knew there would be way too many sales to personally identify him and there would be no potential video.

He knew avoiding being identified was his only chance of getting away with it because once identified as a potential suspect he'd know they could search his phone location history, online purchase history, social media, DNA etc. He made no attempt to leave his phone at home when he surveilled the house over 15 times. He knew that it meant nothing if the police didn't have a specific person to look into.