Could it be that your PC uses TRB140 as interface to access the internet, thus increasing undesired consumption?
Could you try to modify the metrics of network interfaces on your PC, by giving a lower priority to the TRB USB interface and higher to your preferred interface, as in the example below:
Also, have you tried Darkstat to follow the sources of traffic?
As an alternative, you can download TCPdump from Services -> Package manager page in router's WebUI and enable it to monitor traffic on mobile interface for a while in System -> Administration -> troubleshoot page.
Best regards,