Unexpected shutdown

[n j]

Hello,

my FreeBSD (6.2 release) box shut down at approximately 3 AM with no
apparent reason whatsoever. I'm looking for any hints/pointers that
would help me detect why the box decided to shutdown. Perhaps someone
with similar experience?

This is from 'last' command:
shutdown         ~                         Sun Nov 18 03:12

This is from 'messages' log:
Nov 18 03:12:26 my_machine syslogd: exiting on signal 15

The fact that at least this information was logged suggest it was not
a Very Bad hardware crash, especially since the machine booted O.K.
when turned back on and is up now, so it seems it might be
software-related.

I checked ipfw log, apache log, postfix log and other less relevant
logs to find anything suspicious, but nothing was found. I also have
monitoring software (munin) which shows absolutely no suspicious
activity (regarding cpu, load, memory, network usage...) at the time
of shutdown.

What would be your guess - software, hardware, security issue... - as
to what happened?

Thanks and regards,
-- 
Nino