As you do not have a private physical network, the Windows server will broadcast it's presence by default to any other servers on the same network, whilst it's discoverable it doesn't mean that it's accessible, in addition to this whilst you can see other machines, you cannot access them. You can configure the server to prevent it broadcasting the announcement, please see http://www.petri.co.il/hide_a_server_from_the_microsoft_computer_browser_service.htm