Debian – Why or who did reboot the server

One of our servers rebooted last night

reboot   system boot  2.6.32-5-amd64   Wed Jan 15 03:24 - 16:07 (1+12:42)

Our hoster didn't do it, nor did I.

syslog does not show any error

Jan 15 03:17:01 big /USR/SBIN/CRON[840]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly >> /dev/null)
Jan 15 03:20:01 big /USR/SBIN/CRON[844]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Jan 15 03:24:44 big kernel: imklog 4.6.4, log source = /proc/kmsg started.
Jan 15 03:24:44 big rsyslogd: [origin software="rsyslogd" swVersion="4.6.4" x-pid="893" x-info="http://www.rsyslog.com"] (re)start
Jan 15 03:24:44 big kernel: [    0.000000] Initializing cgroup subsys cpuset
Jan 15 03:24:44 big kernel: [    0.000000] Initializing cgroup subsys cpu
Jan 15 03:24:44 big kernel: [    0.000000] Linux version 2.6.32-5-amd64 (Debian 2.6.32-48squeeze4) (dannf@debian.org) (gcc version 4.3.5 (Debian 4.3.5-4) ) #1 SMP Mon Sep 23 22:14:43 UTC 2013
Jan 15 03:24:44 big kernel: [    0.000000] Command line: root=/dev/md1 ro acpi=ht
Jan 15 03:24:44 big kernel: [    0.000000] KERNEL supported cpus:

auth.log does not show anything in a wide timeframe arround 03:24.

lastlog does not show any connectings besides myself.

faillog has no entries.

uprecords

big:/var/log# uprecords
     #               Uptime | System                                     Boot up
----------------------------+---------------------------------------------------
     1   210 days, 06:33:18 | Linux 2.6.32-5-amd64      Wed Mar 13 09:49:06 2013
     2    97 days, 10:58:48 | Linux 2.6.32-5-amd64      Wed Oct  9 17:23:44 2013
->   3     1 day , 12:54:06 | Linux 2.6.32-5-amd64      Wed Jan 15 03:23:22 2014

I can see log entries for what happened after the reboot, but I cannot find ANY information for what caused the reboot.

The CPU usage was very low during the time so it is surely not a overheating problem.

03:10:01 PM     all      0.01      0.00      0.00      0.00      0.00     99.99
03:15:01 PM     all      0.02      0.00      0.00      0.00      0.00     99.97
03:20:01 PM     all      0.01      0.00      0.00      0.00      0.00     99.99
03:25:01 PM     all      0.02      0.00      0.00      0.00      0.00     99.97
03:30:01 PM     all      0.01      0.00      0.00      0.00      0.00     99.99
03:35:01 PM     all      0.01      0.00      0.00      0.00      0.00     99.99
03:40:01 PM     all      0.02      0.00      0.01      0.00      0.00     99.97

Does anyone have an idea where I can go on with the search ?