[Milhouse]

One other thing I've just remembered when the Pi unexpectedly wakes, is that the screensaver (in my case, Dim) is not deactivated, as would normally be the case if keyboard/remote input had woken the Pi. Somehow the "wake" event is firing and re-enabling HDMI etc., yet the screensaver remains active so the screen is still dim until user input deactivates the screensaver.