in order for google to know what is on the web it needs to 'spider' it every now and then...
It starts off with a load of links it already knows about, then goes to each one and caches the pages, then goes to all the links on that page and so on.
So eventually it will see every page on the internet that has a link to it on another page on the internet.
Explained quite well here: http://www.googleguide.com/google_works.html
The forum software recognises the google spider ting from the HTTP headers it sends, and lets it view pages from the perspective of a registered user which is why it appears on the logged in users