If this is the case, then you only need to set a packet monitor between the computer and the ISP router to observe such magic packet.
Or will you claim that all machines that are capable of such tasks are already compromised?
Or will you claim that all machines that are capable of such tasks are already compromised?
Admittedly it's absurdly complicated to do that at global level, but let's say someone in the right place manages to do that, the next level would be doing the same at iron level on computers, so that each subsystem can talk with others and the external world without administration tools noticing, because it's all done through a covert channel set up by closed software. That would be the perfect weapon to build pervasive surveillance that no security software at any privilege level, not even debuggers, would detect.
The only way to find something fishy is going on would be to sniff inter-chip communications locally and set digital analyzers on network cables with appropriate software. Network analyzers could fail if they use the same network chipsets, as would do a normal packet monitor.