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.
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.)
in your case I would scrutinize the task scheduler for tasks that are allowed to wake the PC.
That said, the tablet felt so infuriating to me that I was honestly happy when it was gone again, despite the financial loss.