Startup sound when system should be idle

I have Volumio on Pi4 which is powered and running all the time. It is connected to my audio system, which I also leave powered on.

In the last couple of months, there have been a few occasions when I was working at my computer and I suddenly hear the Volumio startup sound on my speakers. This happened when I haven’t played any music on Volumio for at least a few hours.

If I hear the startup sound, I presume that Volumio rebooted for some reason. Either Volumio crashed and restarted, or a hardware fault, or power interruption (but nothing else was affected). This hasn’t been an issue for me and I wouldn’t even have known except that I happened to be in the room at the time. However, I’m curious about what happened. Is there a log file or anything else that will give me some clues?