A couple times in the last 24 hours I have left my computer (a desktop) on and when I come back several hours later it is off. So far I haven't seen it turn off myself.
Usually if the power is cut the computer will restart when the power is returned, so it probably isn't that because the computer is completely off. If the power supply messes up it will usually not start back up until the PSU power is toggled iirc, which isn't the case.
The CPU is idling at 40C so it doesn't seem like a thermal cutoff.
I can't find anything in the event viewer.
My son likes to mess with the buttons on the front of the computer, but when I restart chrome tells me it was not shut down properly and afaik the power button tells the OS to shutdown and the OS would send WM_Close to Chrome rather than kill it.