I'm one of those people - my computer kept waking up in the middle of the night and that command would always just return "wake source: unknown" for me.
powercfg -devicequery wake_armed
powercfg -devicedisablewake "Device Name"
if you haven't tried this, try it, and I really hope it helps.
in your case I would scrutinize the task scheduler for tasks that are allowed to wake the PC.
It's possible auto wakeup was enabled in the BIOS, with the wake time set to the default of 00:00.
Alternatively, if it was a different time, perhaps *that* was the default, or maybe the value accidentally got changed.
Alternatively-alternatively, maybe the BIOS had a bug in it that somehow enabled auto wake without presenting an option. To tackle this, I would've reset the BIOS settings. (I once had an ADSL2+ modem that randomly started flaking out and not connecting correctly. Changed every accessible setting I could, no dice. Factory reset = instantly back to normal.)
That said, the tablet felt so infuriating to me that I was honestly happy when it was gone again, despite the financial loss.