This picture from your link shows looking into the kitchen on 2nd floor, and one of the victims bedroom on the 3rd:
There's another view from the road behind (Queen wraps around the house) that offers a view into the other bedroom. Someone could easily observe both bedrooms from the relatively secluded parking lot above/behind their house. There is also a very large apartment complex on the other side (east)of Queen from the house.
500 Queen Rd, Moscow, ID 83843 (Google aerial and street view available)
One could easily see residents of any of those apartments being aware of a "party house" with many attractive sorority girls. One could also likely look into their bedrooms from the parking lot above/behind the victim's house. Entry would be steps away to the sliding glass door.
Story time: some years ago, I lived in a house where neighbors had a teen daughter. One night I went to take my trash out and discovered a similar age male (16?) standing up in one of my trees trying to see in her bedroom. I told him to leave in no uncertain terms and would call the cops if I caught him in my yard again. (I never did thereafter.
Bottom line is some guys do that sort of thing. It was even part of an old music video (Jenny 867-5309) where Tommy Tutone gets hauled off for peeping. That was back in the 80's.
Anyway, where I'm going is one or both of the girls on the 3rd floor may have had a peeper/stalker, maybe the guy ran into them in a bar and got ignored. Maybe he waited for them to return. Since the big apartment complexes and other rentals are all around, easy to make his getaway. This would presume he lived alone or roommates had already left for the holidays.
All Moo. Sorry for the long ramble, just another possibility.