Perhaps Abby was abducted or at first left on her own but was then unable to leave. Perhaps she either escaped or convinced her captors to let her go & returned home. But perhaps she isn't able/willing to tell LE or anyone else who took/kept her because of either:
1) Stockholhm Syndrome
http://en.m.wikipedia.org/wiki/Stockholm_syndrome
2) Or she's terrified that if she implicates someone they will come back for her or kill her family (for example, Elizabeth Smart's abductor would often threaten to kill her family if she didn't cooperate). Maybe Abby negotiated her freedom by convincing her captor to let her go if she promised she wouldn't implicate him and he threatened that if she did, she and her family would be im danger. Perhaps what she saw while in the hands of this captor made her believe he was serious and capable of some bad things.
She is only 14. I could see either of these being possibilities...and could explain why the police seem to need the public's help, etc and are still trying to figure this out.
(this is all hypothetical if she was abducted or held against her will)
I am glad she is home and is safe!