Disable HDMI-CEC on your STB set it to standby.
On your TV change the HDMI entry to something else.
Come back to the HDMI entry of your STB and have a look.
You will see this:
screenshot_20181231165700.jpg 32.27KB 2 downloads
So in standby the STB continue to send an image to the HDMI output. This show is someone is streaming from the box in stand-by and then the tuners usage.
I just grabbed the image thanks to OpenWebif. :-)
Pr2