Sonos One started playing spontaneously at full volume this morning - apparently, Sonos hasn't fixed this ongoing issue

Where was the music originating from? 

Sending a diagnostic to sit on the Sonos computer accomplishes exactly nothing, you might as well go out and shout at the clouds.

Also posting personally identifiable information here, like the diagnostic number is prohibited.

However if you send a diagnostic and then CALL Sonos the diagnostic will be looked at and used to diagnose your issue.

Your post title is also a bit off, unless of course you have actually found a problem that nobody else seems to have found. Many report unanticipated sound but unless I missed it all were tracked back to a source other than a bug in Sonos software.

Huh. How did I miss your Sonos Live Radio statement?
 

I’ve never seen such a thing. Did you submit a diagnostic within 10 minutes, to capture what caused this issue, and will you call Sonos support?

The source was Sonos Live Radio, but no one initiated that action in the home.

Submitting a diagnostic and working with SONOS support is the best approach. I can imagine a couple things: A cat fussing with a controller or player. An intermittent issue with a controller that results in executing a SONOS controller command.

 On one or two occasions one of the rooms will start playing something, but not at a loud volume.  I’m a bad boy.  Forgot to send diagnostics / call.  Must admit though calling now is much better as you get connected in a very reasonable amount of time.

Reasonable in some cases maybe, but not in this case.  Support trying to dig into it.

I avoided any chance of this happening here. No cat, voice control, or external service control allowed. Worked for 20 some years now, not one note.

The cat issue is all too common to be ignored, warm Sonos cold cat, and touch controls, not a good combination.

There have been acknowledged cases where a Spotify account remained connected to a Sonos system and the user, back in their own home, increasing the volume to hear their system without realising they were still connected to the Sonos one. 

And that’s why I asked my original question. But it wouldn’t apply to Sonos Radio. So that theory is down. As suggested, the diagnostic within 10 minutes would show where the command came from. I’ve never seen a successful post about it being a software issue, but they change it frequently enough to make it a very slight possibility. Much more likely the command came from elsewhere, but the diagnostic would tell, IMHO…assuming that the data is there, and how long it lasts to be written in a diagnostic, which tends to be why we all throw around that 10 minute comment. I would hope, for memory’s sake, it isn’t infinite.