Are you sure you can do this without network? I'm pretty sure this is not possible. Maybe you had Wifi and 3G disabled. But you say you were snooping the USB connection while you were making a backup. At that moment, the phone uses your PC to get an internet connection too. :P
So if you really want to test if all info is on the device, you should also disconnect your pc from the internet. If you have your phone in airplane mode and your pc has both wifi and ethernet disabled, you will probably get an error if you try to make a backup.
you are correct, I forgot to disable the wlan on phone, so at the start of WP7EaSYbackup, it was able to connect to MSFT. If I disable all, even 3g/2G Connections, then Easybackup stopps working, in the logfile you can see that it could not connect to the updateserver. Nevertheless, I'll investigate soon, what the Updateprocess is talking to MSFT.