My guess is that, the text search functionality searches for the text only in the posts made. And since your signature is not a part of the post technically, it doesn't even bother to look into the text in the signature. If you use the Google search on the forums for your signature text, it shows some of the posts you have made.
Originally posted by Mani Ram: [...]And since your signature is not a part of the post technically[...]
Good guess Mani!
Just a minor clarification - the signature is not part of the post (technically or otherwise). It just gets displayed when you view a topic. I think it is cached in a per-forum basis.
That is, at present forums # 10 (JavaRanch) and 24 (SCJP), and others, have associated "CIAO \n Peter M. Cooke \n scdjws, scea(phase1), scbcd, scwcd,scjp" with user # 69133 (Peter Cooke). So whenever a post from Peter is displayed in those forums, the associated signature is displayed.
If Peter then went and changed his signature, and posted in the SCJP forum, then all his posts in the SCJP forum would display the new signature, since the act of posting will update the SCJP forum's cache of his details. But if he came back to this forum it would still show the old signature since he has not yet updated the cache in this forum by posting here.
The same rules apply for most of the other "non-post" content - your display name, status (greenhorn, ranch-hand,...), number of posts ... are all stored seperately.
If somebody sends you a Private Message, and you respond to it, you will be able to see your response in the "My Profile" link - that response will have a link to your externally viewable profile (the one that has your member number displayed).