Whenever I'm "on to something" I get a panicky sense of urgency. I just got it now. Here's what I think happened to Barbara, and because I am an amateur, could be completely wrong. That's the caveat.
I think, just like RT speculates, Barbara was crossing the road, and I believe she was hit by a vehicle. They panicked and threw her in the trunk, and dumped her somewhere else. That long desert drive to Vegas, they could have been looking at scenery, reaching for something, or whatever, (on cruise control), and didn't see her.
That's what I think happened to Barbara.
LE should look for signs of this on the highway, if they haven't already.
Amateur opinion and speculation