If you want to make sure to get all objects of your network, you can import them directly from the Active Directory. Regardless if they are on or offline, you will import everything that exists or existed on the domain. All the other search methods require the computers to be online at the time the search takes place.
If you activate this search method, the pool of unassociated computers shows the OU structure of your domain tree with contained computers and servers. You can limit the import by entering name fragments that will filter out anything matching. If you filter “ou=_inactive”, inactive objects of the data base won’t appear in your pool. You can separate multiple fragments by comma.