An update from CPS Germany.
"Dear Mr. Burton,
Thank you for your answer.
Actually, we received some feedback from other customers over the years with the same issue. This happens then when only the TC is to the camera attached. In this case, you have an “open circuit” and the camera will keep trying to communicate – even is the camera is switched of – through the TC with the Lens, which is not attached. This will happen not with all gears, but from time to time, we receive some feedback like yours. The ON/OFF switch works not a real switch, but it is actually a “software-switch”, so the camera can continue communicate with the other components.
In these cases, we recommend to attach also the lens to the TC. Due to the length of your gear, this will not the best option for you. Either you take the battery out from the camera, or you attach the TC direct to the lens, without the camera.
You can also try to reset the camera to the factory settings (clear all camera settings).
We hope to have helped you."
So, we are aware of the problem and can do/will do nothing about it.
I'm not convinced there isn't a problem with the new LP-E6N batteries. The first time I charged the battery that came with my 5DIV, it discharged itself completely overnight, left in the camera (135L attached, camera turned off). Since then I've left the battery out of the camera, but I went to use it today and it's down to 8% (from ~70%) in a week. My 6 year old LP-E6 does considerably better than that.