How can you see from the logs why Linux is rebooting?

Question

linux

OpenSUSE Leap 15 is available, it initially had an automatic installation of updates with a reboot.
I disabled the rebootmgr service, set the policy in /etc/rebootmgr.conf not to reboot.
In the morning today I found a laptop in limbo (I have encrypted partitions, so nothing will come up after a reboot without a password).
How it is possible to define it on broad gulls? journalctl, as far as I understand, only stores a record starting from boot, I can’t see what caused the reboot and when it happened exactly. The only thing I could find was the time when the system was updated, but again, there is no confirmation that the reboot was made immediately after it.

Answer 1

journalctl , dmesg