[ajxs]

If you have firmware level access to a device like an NIC, you could theoretically circumvent the NIC reporting any network activity at all from your actions. This wouldn't cover external network monitoring of course, but how often do you scour the packet logs of your router's I/O?

[tinus_hn]

For it to remain a secret it has to be noticed by nobody. I do not regularly scour packet logs but you can be sure people exist who do.