Only what's posted in the public forums can be found on a search engine.
If you're wondering which forums are "public", here's the answer from the FAQ's thread at the top of this forum:
Which forums are public?
Some of the forums on the board are public, which means that anybody can read their content, no matter whether they are registered or not.
Public forums include: Wonderful Threads, Research, The Fun Forum, Board Guidelines, Suggestions, questions, and complaints
Secondary survivors are restricted to the Healing Together, Announcements and public forums.
Also, here is this notice that is pinned at the top of all public forums:
Public forum notice
Please note that this is a public forum open to guests. Your username, the content of your posts, and your homepage (if applicable) will be viewable by non-registered guests. Your signature, profile, and contact info (including email and messenger screennames) will be viewable ONLY by registered members.
So, for example:
You posted this question in one of the public forums. Therefore, non-registered people can see and read the content within this thread, and they can also see our usernames. However, they CANNOT see your profile, signature, or contact information. They also CANNOT see what you've posted in ANY of the private forums(my voice, my story, etc etc)
Furthermore, if you had posted this in my voice on the other hand, only registered members would be able to see what you've written.
I hope this clears things up a bit!