A very troubling case. She does not seem to be the type to run away, however people can and do hide many things! I would be very interested in hearing about what can be found on the computer, did she have 1 in her room? Since she did not have close friends there, did she find friends on-line?
If she was very reserved/shy, it's hard to picture her just taking off on her own.
Why did this have to happen when her mother was away? Was it so she could leave with a car? If her father looked in at 5:30 a.m. he must leave the house very early for work, was there someone in the neighborhood who would know she would be alone in the house after he left? She could have written the note but been forced to do it. Did she have access to much money and was it gone also?
Finding the car 5 blocks away from her body and unlocked sounds to me like whoever left it did not care what would happen to it, maybe they were thinking it would be stolen.
Also, what items of hers are missing, did she pack a bag? A teen-age girl running away would have many things she'd want to take with her and from the sounds of it, she would have taken several pictures of her dog.
I hope this is able to be solved.
VB