I might have finally found it.
Windows 10 has itself scheduled six ways to Sunday with various reboot and auto-update cycles. After disabling damn near every scheduled task and deleting everything in every folder containing the word "update" … I MIGHT have gotten rid of it. Apparently there was an end of service schedule in there, too.
What is noteworthy is that there is a task for getting a Windows key. A friend of mine in the tech field noted that some Win10 computers will un-activate themselves. These computers are not connected to the internet.
Win10 seems to use an "account key" to go online and retrieve a time-restricted software activation key that expires after around 30 days. No one notices because we usually don't take our computers offline for that long…. But in his line of work, most of their computers are not on the internet and they have encountered activation problems with Win10.