This took a while to track down, but here it is: with automatic updates turned off, obvious scheduled tasks disabled, and even a few services experimented with, Media Center turned out to be the culprit in my case. It's possible that others might run into different issues, so here's how to go about diagnosing this.

1. powercfg /waketimers. /lastwake wasn't useful at all, but wake timers turned up the service that was waking up my machine:

2. This did not show up under Services, but apparently it is a scheduler task... which you have to dig for a bit in the Task Scheduler UI.

Go to Task Scheduler → Task Scheduler Library → Microsoft → Windows → Media Center to find it:

3. Right-click and disable with extreme prejudice:

This took a while to track down, but here it is: with automatic updates turned off, obvious scheduled tasks disabled, and even a few services experimented with, Media Center turned out to be the culprit in my case. It's possible that others might run into different issues, so here's how to go about diagnosing this.

1. In an elevated command prompt, run powercfg /waketimers.  The /lastwake option wasn't useful at all, but /waketimers turned up the service that was waking up my machine:

    C:\WINDOWS\system32> powercfg /waketimers
    Timer set by [SERVICE] \Device\HarddiskVolume2\Windows\System32\svchost.exe (SystemEventsBroker) expires at 2:36:15 PM on 11/3/2015.
      Reason: Windows will execute 'NT TASK\Microsoft\Windows\Media Center\mcupdate_scheduled' scheduled task that requested waking the computer.

2. This did not show up under Services, but apparently it is a scheduler task... which you have to dig for a bit in the Task Scheduler UI.

Go to Task Scheduler → Task Scheduler Library → Microsoft → Windows → Media Center to find it:

3. Right-click and disable with extreme prejudice:

This took a while to track down, but here it is: with automatic updates turned off, obvious scheduled tasks disabled, and even a few services experimented with, Media Center turned out to be the culprit in my case. It's possible that others might run into different issues, so here's how to go about diagnosing this.

1. powercfg /waketimers. /lastwake wasn't useful at all, but wake timers turned up the service that was waking up my machine:

2. This did not show up under Services, but apparently it is a scheduler task... which you have to dig for a bit in the Task Scheduler UI. Go to Task Scheduler > Task Scheduler Library > Microsoft > Windows > Media Center to find it:

3. Right-click and disable with extreme prejudice:

This took a while to track down, but here it is: with automatic updates turned off, obvious scheduled tasks disabled, and even a few services experimented with, Media Center turned out to be the culprit in my case. It's possible that others might run into different issues, so here's how to go about diagnosing this.

1. powercfg /waketimers. /lastwake wasn't useful at all, but wake timers turned up the service that was waking up my machine:

2. This did not show up under Services, but apparently it is a scheduler task... which you have to dig for a bit in the Task Scheduler UI.

Go to Task Scheduler → Task Scheduler Library → Microsoft → Windows → Media Center to find it:

3. Right-click and disable with extreme prejudice: