I don't know how to phrase this, so I hope it won't come out wrong.
I'm wondering if exploring the possible reasons why Anna has not been found or has not got in touch yet would help in planning the strategies for future searches and sleuthing (in terms of what to do to overcome the possible obstacles).
Going on the assumption that Anna is alive, here's what's in my mind as some possibilities:
1. Anna does not remember anything from before the abduction, the abduction itself, and the time immediately after the abduction. Hence she doesn't know her family is looking for her.
2. Anna knows the people who brought her up are not her biological family, but has been told something convincing enough so she isn't even looking to find her biological family at all. This includes stories of parents dying/abandoning her, being a foundling, a fatal accident... What she has been told is convincing enough for her to not question lack of photos of her before age 5 or 6 and any lack of documents.
3. Stockholm syndrome.
4. Abductor still alive and in control of Anna in some way; coupled with point 3 perhaps. She doesn't have much of a chance of doing any research perhaps.
5. Anna is somewhere where she doesn't have much contact with the world, little or no internet access, doesn't get to go out among people much unless absolutely unavoidable. A lonely farm in the middle of nowhere perhaps, a more-or-less self-sufficient existence.
6. Anna is in some closed community, possibly religion-oriented or a sect, so a lot from point 5 might apply here, too.
7. If she was taken out of the US, she could be somewhere non-English speaking. Five is young enough to forget mother-tongue. Her age now could mean that she only speaks the language of the country (depending on the country and its education system, plus her circumstances), or that her English is limited and so not much use in searching for her roots. This could also mean she might be searching desperately for her family, but in a wrong country, on the wrong continent, in the wrong language.
I'll stop here for now. I wonder if anyone has any thoughts on this.
If anything here is offensive or causes pain, I hope mods will delete this post.