In terms of cases where people are either found right away or not...this case is very new, really. Just seems a very long time.
It is not that I believe LE is holding back much in this case, other than info about the letter, but that I do not think they have much else. They have never been able to establish, from what they say, whether she was taken by force or left on her own. If the letter told them she chose to leave, that does not help them much, I guess, as clearly a girl her age is not meant to be out on her own. I do not think they would want it known if in fact Abigail wrote to say she chose to leave, because it does not change the fact that she needs to be located.
Still, if she did leave and is not being "held", then it changes where she might be spotted, in whatever city she is in. She might use services such as food kitchens, church charities or the YWCA (if they still exist.) So it could be useful to know.