I tend to think starting at the very top in Jason's case means trying to answer not the "who" question (i.e., was a work colleague, another acquaintance, or a stranger responsible) but the "how" question. I mean, regardless of who had a hand in his disappearance, I'd like to know, first of all, did he (a) voluntarily/intentionally walk into whatever fateful situation occurred, or (b) get involuntarily diverted from his ordinary intention of showing up at the high school and catching his ride to work?
I think most people seem to assume some version of (a), but I tend to start out from (b). Maybe the reason (b) isn't a the most popular starting place is because it conjures up the notion of a snatch-and-grab sort of abduction, which I agree is entirely implausible. Another non-starter, in my opinion, is the scenario (sometimes mentioned about Jason's case) where a panicked reaction to an automobile accident (i.e., he got hit by a car) leads someone to conceal his body. Why not just hit-and-run? But the kind of think I think of is hoodlums in the back of a truck smashing things with a baseball bat (unlike an accidental collision, in this case it might be reasonable to dispose of the body) or a sniper or serial shooter (although those criminals usually don't want to get near the victim's body).
