How to determine cause of reboot

Hi, I'm currently running a test box and it rebooted yesterday for no specific reason. What steps should I take to diagnose this? There's nothing in /data/crash.

I had been tracking the ARC usage, and it had recently hit the max. Perhaps an overload of the ARC? I'm using autotune.

BuildFreeNAS 9.3-BETA 2014-11-12 14:34:09 GMT
PlatformIntel(R) Xeon(R) CPU X5560 @ 2.80GHz
Memory24542MB
System TimeSat Dec 06 21:48:43 EST 2014
Uptime9:48PM up 22:25, 0 users
Load Average0.09, 0.08, 0.07

Thanks