With a tip of the hat to the late Rob Grill and The Grass Roots ... (video)
I previously reported finding my Mythbuntu PC, used strictly for recording TV shows, up and running occasionally when no recording was scheduled. Further investigation revealed that phantom wakeups always occurred at midnight UTC, and only when nothing was scheduled. (If a recording was scheduled later in the week, the PC would sleep through midnight UTC without problem.) I've finally identified (and fixed) the problem, and it has nothing to do with MythTV.
The BIOS (a somewhat dated Phoenix Award Workstation BIOS) contains a flag in the power settings labeled "Resume by Alarm", alongside entries "Date (of Month)" and "Time (of Day)". The "Resume by Alarm" setting was enabled, while the "Date (of Month)" and "Time (of Day)" settings were zeroed out. I don't recall enabling the "Resume by Alarm" setting, but the machine originally came with Windows Media Center (home version) installed, and it's possible WMC requires "Resume by Alarm" to be enabled.
The instructions for setting up ACPI wakeup in MythTV mention "fussy" BIOSes for which one might need to disable real-time clock (RTC) alarms in order to let MythTV wake the system. In my case, it was not necessary -- wake-up went fine -- and so I assumed, as it turns out incorrectly, that my BIOS is not "fussy". It also seemed to me (again, incorrectly) that in order to have the system wake from power-off when an alarm went off, one would need "Resume by Alarm" enabled.
The problem is that "Resume by Alarm" and the adjacent date and time features are designed for specifying a recurring alarm (e.g., wake at 8:27 AM on the 20th of each month, until the alarm is cleared) and (here's the kicker) "Date (of Month)" equal zero means not that there is not scheduled alarm but rather that the PC should wake at the specified "Time (of Day)" every day of the month. Since "Time (of Day)" was 00:00 and the BIOS uses UTC, this meant wake at midnight UTC every day, unless a different alarm (for an as yet unrecorded program) was set. When the last scheduled recording was done, the BIOS went back to wake-daily-at-midnight mode.
The fix was simply to disable "Resume by Alarm". Alarms set by MythTV still work, and now when nothing is scheduled, nothing happens.