I had this problem once before when I was running Windows 7. I don't remember which device was causing the insomnia but I remember running the command
powercfg -lastwake which told me exactly which device caused the wake-up. But the same command under Windows 10 only gives me this useless response:
Wake History Count - 1 Wake History  Wake Source Count - 0
Any other commands I could use to find out exactly what caused the wake-up?