Unless you took a backup I guess it's not relevant anymore but if it happens again you can narrow down files last changed around some timestamp like so: find /var -mmin +4 -mmin -5

Which logs did you look at already?

If you're using journalctl, I think you should have shutdown messages in the log. You might need to filter by the previous boot for them though (https://linuxhandbook.com/journalctl-boot-logs/).

For dmesg, you might have old, rotated logs from previous boots in your /var/logs folder.

I'd expect any logs around power management to end up in one or both of those places.

You could also try manually triggering a suspend or hibernate to see what happens. I remember having a machine that would suspend fine, but if it was suspended too long, it would hibernate. And for some reason it didn't know how to come back up after a hibernate.

I think I'm just going to keep an eye on it and see if it happens again, since this is the first time it's ever shut itself down unprompted like this. My cat has also figured out where the power button is and likes to press it occasionally, so unless it happens again I'm just going to chalk it up to that. Might replace the PSU just for good measure, since it's getting kinda old and was made by EVGA, about whose reliability I'm entirely ignorant.

When this happens to me I mostly assume Linux shutdown automatically because of a critical over temperature event. I've seen it in the logs a few times but I don't usually check anymore.

There's an example of this here https://unix.stackexchange.com/questions/502226/how-do-you-find-out-if-a-linux-machine-overheated-before-the-previous-boot-and-w

Does it happen at a consistent time or frequency? Like at 8pm or after 2 hours of being turned on?

The last few weeks I've had a similar problem and I can't find a solution. Same installation of manjaro for around 7 years. I made no system setting changes. All I do is update the system with the package manager. When I check some systemd logs or dmesg it all looks normal except from wake it mentions something about an unresponsive disk, but it always works fine after the second wake or if I just shut it down and then boot it up. It's only a problem from suspend to wake. I can type in my password and log in but it pretty much seems after 10 seconds it suspends itself again for the first time.