Analyse data on USB line
There is comms happening all the time over USB, StartOfFrames if nothing else.
The best way to see what's going on is to use a protocol analyser such as a USB Beagle from Total Phase (I have no affiliation, but own, use and am happy with their tools).
Wireshark can be used and there are various other software tools which can do USB packet capture, but I find none work quite as well as a hardware tool.