As more info comes in, it appears there in fact WAS a suspicious red truck seen by a local woman named Welma Robinson (also known as Robin/Robinson Ordway, online) at the Stage Shop around the time of Maura's disappearance. I now believe her statement that this red truck had Massachusetts plates.
The confusion comes from other red trucks that were spotted or checked by police officers following Robinson's initial report. The Glynn brothers' truck being one of these.
I believe the evidence in this case suggests Maura may have been traveling in tandem with another driver or met someone up in New Hampshire, and it was that other driver who picked her up after the accident. This red truck with Mass plates is very interesting. Did Robinson witness this other driver turning around to return to the scene of Maura's accident?
Who did Maura know from Mass who owned a red truck in 2004?