Double-check the wiring for any aftermarket gizmos; that was the culprit in a neighbour's car. Some kind of alarm gadget that had been connected to "always on" power instead of "accessory" or "ignition". Apart from that... there's just so many things in these newfangled (post-carburetor) cars that can develop faults. If you have access to an infrared camera, maybe that would reveal something that's warmer than it should be.

 

Some cars now have a fibre optic communication system in addition to to several CAN bus networks.