Aside from checking to see what's running in the background, check to see what hardware pertaining to networking the OS believes it has running. (start -> settings -> ctrl pnl -> network)
if you have something listed in there, it is more than likely the OS is trying to use it for a network connect. Some video tuners will place them selves in your network stack and bind a TCP/IP to them for various reasons. There are other devices that will bind networking protocols to themselves.. Worth a look.

Regards.

edit :
There is no way that it could have communicated to anything, that is why it is wierd to me.
Your lack of cabling is what causes the error. Your machine will initiate a DHCP request, hence the broadcast packets on 255.255.255.0. Should there have been sufficient hardware, cabling and respective DHCP servers, your system would have acquired a new DHCP lease and your system would have notified you of that as well.