The Cube was attached to the keyboard dock, in which there was an USB Stick and Microsoft Wireless Keyboard Dongle were attached. Could it be the last one that was responsible for draining the battery? Strage though…
Check your power settings, are you sure you turned it off? you might have your power button to act as “sleep”, this will still consume battery power, to switch it off either select the shutdown option from the taskbar or edit your power options and change the power button setting to switch off instead of sleep.