This subject has been raised recently and I have asked HU for clarification. Please see their response below:
All posts appear on search engines, regardless of whether or not they are Community Only. However, as mentioned in this article (http://support.healthunlocked.com/customer/en/portal/articles/1598083-how-do-i-create-a-post ) posts that are Community Only have only the title and first line visible to people who are not logged in members of that community.
When accessing that post from Google, you should find that you can only see the title and first line of the post (unless you are logged in, in which case you will see everything).
Please note - if you are unhappy with this information, your concerns and/or complaints should be directed at firstname.lastname@example.org and NOT Thyroid UK and/or the Admin Team - thank you.
Our suggestion would be to make sure that your username does not identify you and that your title and first line of any post are similarly anonymous.