Thinking outside the box
China is well known in the industry for getting counterfeit chips into the supply stream, so it is not unfathomable that they could have introduced a counterfeit version of a device that is in the design of the motherboard already - maybe the Flash memory used for the BIOS with some hard-coded instructions built-in. No need for special traces on the board for power or data; and board level X-rays or IR scans would find no rogue device embedded in or on the circuit board. Chip level X-rays might detect differences in the silicon, but that might be fool-able as well. Obviously they would have several technological and procedural hurtles to get over to accomplish this - but a determined, state-sponsored effort might be able to do this.
Just thinking outside the box on how it might be done. I'm not conjecturing either way on if it actually was done.