I feel that Abby's birthday being the same week as her disappearance could be significant to her state of mind.
Being that Abby recently started high school, and was turning another year older that week, she may have been starting to feel very "grown up". Even her Monday cleaning job probably contributed to a sense of independence and feeling more adult.
I remember on my 16th birthday listening to that Five For Fighting song "Fifteen" and feeling SO old and wise (what a laugh now, looking back!). There is a very dramatic feeling to most life events for a lot of girls that age. I remember constantly comparing my life to the teen dramas on the CW and popular pop songs-- in reality my life was completely average and dull.
Maybe Abby left in search of some excitement. Maybe the idea to leave (possibly to follow her dreams? Thinking back to Zenya's letter) even blossomed from something she saw on television. I would love to know specifically what books, TV shows, and music she was into. Here on WS there have been several cases of young people who have loved the book Into The Wild disappearing or dying when trying to take their own Into The Wild type journey. I do not think Abby wanted to go into the wild for a rugged adventure! But perhaps her journey was originally inspired by something as well.
Just trying to gain some perspective into Abby's state of mind on October 9th.
FWIW, I believe Abby left willingly but her plan derailed and she is no longer out-of-contact by choice.