I have a Philips Hue sync box which I used to have set to be powered on by HDMI activity from my old BT box. The new BT box seems to randomly send output via HDMI, even when switched off which turns the Hue box on.
This is likely due to HDMI-CEC or ARC which is how the STB can control the TV volume and source choice and I don't know of a way it can be disabled on the STB completely but perhaps try changing -
Settings -> Bluetooth Remote -> Control Volume & Source -> Change from TV to Set Top Box