How protected is hidden content?

I have subscribers who wish to have a private message and discussion center. Some are concerned that Google spiders will find messages in hidden content.

Therefore, the questions:

1. Is protected content in Membership, or in BP Groups, or any other method really protected from the outside world?

2. Can Membership or anything else totally protect a sub-blog?

3. Alternatives are images or PDF files in folders hidden by Membership. This is high overhead. Is this the only way to protect group communications?