ReadyNAS Ultra 4+ silently shut down

My Ultra4+ is runing RAIDiator 4.2.20.  This morning I woke up to my NVR disk alarm going off.  It turns out that the NAS had silently shut down sometime over night.  I powered it back up and it booted normally, no complaints or fsck.  That would seem to indicate an orderly shutdown.  I downloaded the logs, but didn't see anything shutdown related.  The NAS is connected to a UPS, so I guess it possible that we briefly lost power overnight, but there were no tell-tale flashing clocks around the house. :)

The NVR is continuously writing to the NAS, so I would think if there was an abrubt shutdown that an fsck would've been necesary.  If the shutdown was due to a UPS signal I would've thought I'd have seen that in the log as well.  Any ideas on a cause or where else to look for clues?