Why did my computer wake me up?

Tonight, at 02:36 AM, my bedroom computer (Windows 10 1511) decided to wake up from hibeation.

Obviously, my first instinct was to yell at it for waking me up in the middle of the night (it's not a "silent" PC). Since my wife was still asleep, what I actually did was to shut to down (fully) and worry about it later.

Later is now, and I'd like to find out why, oh why, it decided to do such a thing at such an impossible time.

I've checked the following:

Power settings: "Allow wake timers" is set to "Disabled".

Event log: The wake source is "unknown".

How can I find out what caused this madness?