That is an interesting question and probably only lightly touched upon before. As with a lot of things with this case, we have to simply assume a competent police force would have checked databases in regards to Maura's dissapearence. By now, I would think, had her body been found elsewhere it would have been cross referenced and linked to NH.
However, for Maura to accept a ride from a stranger and wind up in somewhere far, like maybe California, that would require a tremendous amount of trust from someone she doesn't know. Even if this was some sort of master manipulator, at some point you'd think Maura would have fled when she felt uneasy.
Of course, a chain of events with numerous people could have landed her far, far away. But then I think we're entertaining the idea she purposely went into hiding, which I don't think is likely.
Still, if this was a bet, my money would go to her being deceased. To a lesser extent, I think her remains are likely in the area of the crash. If she were to be found deceased (or alive) outside of New England, I'd be down right shocked.