Member Rara Avis
Another common name for forums is bulletin boards, and the analogy is pretty apt. If you put something up on the bulletin board at work or at your local supermarket, there's an implied suggestion you want it to be read.
The Internet is a VERY public place. With the exception of our MC section, all of our forums are open to the public. People don't even have to register to read here. And, yea, that's going to include search engine spiders.
The key to anonymity, like the key to keeping any other secret, is to not reveal personal details to anyone. On the Internet, you simply can't whisper softly enough to keep something secret. If you say it at all, it can potentially be heard by everyone who wants to listen.