The "guests" you see in the "Who's online" section of the front page shows spider bots indexing the areas they can reach. These bots are sent by the various search engines. Forums block sections of the forum from these non-member guests for various reason, the most common being the nuts-and-bolts sections where the software lives and administrative functions are done, and the PM transmission/reception areas.
https://en.wikipedia.org/wiki/Web_indexing
https://en.wikipedia.org/wiki/Web_indexing