Otto is of course correct about fitness watches. They collect data (stored on the device for fairly long periods) about heart rate. I am wearing one right now (Apple watch). I can go and look at my heart rate for a 30 day period (easier to use the app on my phone, but the data is on my watch). I can't remember which other crimes involved fitness watch data, but there are some in the crime annals.
So if even one of the four victims was wearing an Apple watch or Fitbit, that would give a clear ToD for that person and therefore an estimate for the others.
***SPECULATION AHEAD***
Your post made me wonder about a lot of things. I too believe the house was watched. So, we can call that Peeping Tom behavior. In this scenario, the perp has a sexual motivation for this fixation and behavior.
I believe the perp either had access to the house before this fixation was developed OR that he went into the house (perhaps often) after the fixation developed. So now we have a form of stalking bordering on burglary (and we have no clue if this perp took things from the house - the residents themselves might not have noticed),
Perp may then have tried to ingratiate himself into the lives of the people in the house, attending their parties, etc. College students can be quite snippy to people they regard as problem people (which is how this guy may have appeared - not a student, perhaps more than a little "weird" from the POV of the young people in the house). The fact that the house was accessible (and observable) almost 24/7 is part of this case.
The fantasy "love affair" the perp was having with one or more of people in the house was destroyed by actually meeting them and being rebuffed. In this scenario, the perp suffers from a not-so-rare mental condition where they flip on people, going from love to hate in an instant. It's often bizarre and brutal to witness. It's often seen in stalkers.
Perp may not have seen the two downstairs residents often or at all. Perp may have regarded the 4 victims as a kind of "team" who had purposefully excluded him from all hope of human happiness, as he was fixated on one of the them (one of the young women would be the best guess). If in the perp's mind he had an unrequited fantasy relationship with K, then K's departure from the house would have triggered the perp, perhaps inducing actual psychosis, and when K returns briefly, he acts out his psychotic worldview. Psychotic people are often capable of being organized enough to hide the trail of their crime, but if this theory is correct, the perp will have spent many hours over many days thinking through what he wanted to do (because the love fantasy is now replaced with a revenge fantasy and he can't function without his fantasy life).
If this scenario is correct, it's possible that some of the roommates had noticed the "weirdo" inside their house or even asked him to leave if he was inappropriate (likely) at parties. And the odd thing is that many times, young people do not feel the need to be super-vigilant about such people, idealistic as they can be. I know I never thought much about the guy who moved into a spaced outside my room in a large shared house. I mean, I didn't like it at all, it was a covered and screened porch space and there he was, night after night, mostly just sleeping there. I mentioned it only to 1-2 people at the time. It seemed normal, I assumed the guy was a student. I knew people who were living rough while I was at uni, I figured he was just a clever one of those. The house I lived in did not lock its front doors, we all just locked our room doors. It was and still is open 24/7 for anyone to come in and out (except for major vacation periods).
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