Situation One
This Linux server is a virtual server. If we reboot it though its virtual software, there is no relevant logs in the /var/log/messages.
Situation Two
This Linux server is a member of a pacemaker cluster. If the pacemaker cluster software fences this server for protecting the whole cluster, there is no relate logs in the /var/log/messages.
Situation Three
This Linux server has critical problems in its system or hardware. Core panic of Linux and hardware problem both can reboot the system automatically without any reboot logs in the /var/log/messages.