For example. The breakdown tells you the following. I did not show real users.
GAGuidestone79 Mon Jan 18, 2010 12:52 am Viewing who is online
MSN [Bot] Mon Jan 18, 2010 12:52 am Reading topic in Europe
Yahoo [Bot] Mon Jan 18, 2010 12:52 am Reading topic in North America
Google Adsense [Bot] Mon Jan 18, 2010 12:51 am Viewing private messages
Why is a [bot] viewing private messages?
They also post and reply to threads.
It is far more likely you are just seeing incorrect displays for the bot. The bot has no way of bypassing normal password security in phpbb (the forum software). In fact anybody could fake a known bot but it wouldn't get them anywhere (other than possibly being able to read paid for content on subscription sites as the site thinks you are just indexing it).
In fact after a bit of digging, the status of the bot (or guest user for that matter) is updated everytime it tries to do something on the board even if it doesn't succeed - in reality the bot will either be sitting at a login prompt or a not authorized message. It will not be able to do anything it shouldn't.