Scans a multisite or multi-network installation to identify all plugins that are active or not.
That menu item will only appear in the Network Admin area for the network with an ID of 1. If you are running a multi-network installation and you activated the plugin on a network other than the first, you won't see that menu item.
No. If you need to see the activation status of plugins in a standard WP install, you can simply go to Plugins -> Installed Plugins in your admin area. This plugin is specifically developed for multisite and multi-network installs of WordPress, where it's much more difficult to get a clear, accurate picture of which plugins are active and where they're active.
When a plugin is installed and activated, WordPress uses that file path as the indicator that the plugin has been activated, and stores that information in the database. If a plugin file is removed or renamed after it's been activated on a site, WordPress doesn't know that it has to remove that old path from the list of active plugins until you visit the Plugins page on each site where it was active.
To make a long story short (too late!), those are plugins that are still considered "active" by WordPress, but no longer exist in your