From Online Manual
At the end of the forum index, there is a section called Users Online. This is a list of users who have recently used the forum. This feature can be enabled or disabled, and the definition of "recent" changed from Features and Options in the admin panel.
This list is ordered by most recent activity. Members who belong to color-coded membergroups (such as administrator or global moderator) will be color-coded on this list. Administrators, are shown in red and global moderators are shown in blue by default. Regular users appear in the default color used by the forum theme.
To learn more about what users are doing, select the Users Online icon or the link below the heading to display the Who's Online page. This page is a list of everyone (guests, members, and hidden) currently browsing the forum along with the last thing they tried to do on the forum. For example, this is the Who's Online page for simplemachines.org.
Usually, you can see in the Who's Online section some users (or even guests) which action is displayed as "Unknown Action". You may found this strange and suspicious but it's not at all. This action appears when you have a Modification installed in your forum and this modification doesn't declare itself properly, so the forum doesn'r recognize what is the user doing.
Nothing, or nothing you can see...
Also, is very common to see the "Nothing, or nothing you can see..." action. In most of cases, as regular user you may see many times this action if you n avigate to the Who's Online section. It means that you don't have permissions to see what an user is doing, for example, if they are posting in a board that you can't see. But also, as an administrador you can also see this action in regular users. In this case, it means that users are trying to access a deleted message or topic or other pages that doesn't exists.
Why do I see my guests viewing threads in Who's Online when they cant view the forum?
Is it possible to avoid Guests view the forum by unchecking the "Allow guests to browse the forum" in Admin > Configuration > Features and Options > General. If you uncheck this options, guests will not have access to any part of the forum, and they will be forced to log in or register. Even with this feature enabled, some guests will be viewing threads in the Who's Online section.
The reason are Spiders. Spiders are internet bots used for web indexing (such as Google or Yahoo) and they will be accessing your threads normally to index them. Is it possible to know how many spiders are currently in your forum. To obtain this, you must first enable the "Search Engine Tracking" feature placed in Admin > Configuration > Core Features. Once it's enabled, you can navigate to Members > Search Engines > Settings and change the "Show spiders in the online list" according to your needs as well as configure other aspects related to Spiders.