Diagnosing a random reboot

Hi all,

I don't beleive FreeNAS ever reboots itself without permission, right?
I've not configured scripts to do so - but when starting a PBI on the system just now, the machine rebooted itself.
Which log file should I be looking at to find the cause of this? Not ideal.

EDIT
HP Microserver N40L, 8GB ram, FreeNAS 9.2.1.6 with 4 plugins (Couchpotato, Sickbeard, Crashplan, SabNZBD)