Since you've been gone

I am reading the book called Since You've Been Gone by Morgan Matson. This book is about two girls and they are best friends. Sloane ends up going missing and makes Emily's life very interesting. When Sloane goes missing she leaves a to do list for Emily. In the book it says,"The list arrived after Sloane had been gone two weeks." The things on the to do list were so random. Something that I was wondering is why she didn't tell Emily that she was leaving. If I was running away I would at least tell my best friend.