I read years ago in Alabama that a pizza delivery man (if I recall correctly) disappeared, car and all... At first, after an exhaustive search, nothing was found...
Then, some time later (I think a couple of weeks), they found the car with the driver inside... It had gone off the road, down an embankment, and ended up where it couldn't be easily seen...
Knowing she had epilepsy, I think this scenario (probably in the water) is the most likely... I'm guessing that at whatever point she disappeared, someone at the restaurant could remember her talking to someone... That's not a good enough reason to think anything was wrong...
Of course it's possible, but it's really just a supposition, since all we know is that she disappeared and so did the car... nice young girl...
whatever
rest in peace