OK, I.ll give it a try.
As I mentioned I have 2 Mutants HD51. When I turn off (put in standby with the remote) the one that works, the little screen oif the HD51 dims and shows time and date. Afterwards I power off completely the TV and the box by pressing the switch of the multiple socket adapter (jack plug) and the screen of the box turns black as expected.
When I follow the same procedure with the second box and put it in standby using the remote, the screen does not dim and instead of showing the date and time it shows, without pressing the menu botton at all, the text Sleep Timer (which as I mentioned earlier is not even enabled) and it stands there even several hours later until I power off the box by using the swithch of the second multiple socket adapter. At that time of course the screen turns black. When I power the system on again the screen on the HD51 shows again the text Sleep Timer and when the channel (the one I was viewing when putting the box in standby the nigth before) turns on it shows on the TV screen in the left corner the pictures of the channel combined to the OpenPLI Menu set to "Sleep Timer" although again I have not pressed the menu botton at all. When pressing Exit a couple of times I get access to the full picture of the channel and the screen of the HD 51 shows the name of the channel.