Steve may be able to better answer your question than I but I believe the non-hidden spiders include well known names like Google, Google Adsense and Yahoo. A hidden spider may be a lesser entity that is using a crawler to crawl through the forum posts to collect information to enter into a database. As an example, that database may be searchable, much like the Google and Yahoo search engines. As a Moderator, the hidden spider is identified in what I see as a Guest.
The word "hidden" does not mean or imply that something funny is going on.