And Laundrie. But:
It would have been hard to have heard absolutely nothing about these murders, even if you're not a news junkie. As a parent of students in school many states away, when I hear about murders anywhere in the vicinity of them, I immediately look it up. I do the same for anyone I know, really, and I'm not particularly honed in on tracking crime, just more general interest/concern about what's going on in areas people I care about are. Surely the parents knew WSU was near Idaho. And if you're the parent of a 28 yo male who needs company for the drive back home, I'm guessing you might need to be a parent who frets over their child.
That said, I can't imagine any parent leaping to, "I wonder if my man-child PhD student horrifically murdered those college students," unless they have a specific reason to do that. So, w/o knowing any specifics or history, not sure I'd jump to think they must have thought or suspected anything.
I ran a few numbers on driving vs flying. Est 850 RT to drive (regular gas; also add motel/hotel stay + food, so call it $1100-1200). RT flights from Pullman to scranton are expensive (~1500, based on the two date sets I pulled (January and June; didn't go searching for historical data and next December was too far out to get pricing using common flight booking sites)), and then you'd either be borrowing mom & dad's other car while there, or renting a car, which is about $50/day in scranton right now (and break is at least a couple weeks). So though I went looking for confirmation that driving made no sense, if money was an issue [seems likely], the quick data I found suggested that driving was much cheaper.