So no alarms setup in your Sonos setup?  In similar cases to this, this is a 3rd party app that is triggering the playback through API.  Best way to troubleshoot is to submit a diagnostic soon after it happens, and then provide to Sonos support.

Is this a single speaker Room or is this speaker part of a stereo pair or surround system?

I would suggest that an alarm has been set, but this is easy to verify.

Do you use a voice assistant such as Alexa?  If so, have you checked if there are any alarms set in the the associated app?

Thanks for the comments, I don’t see an alarm set, or anything on the third party app. I’ll delete the radio station app which is playing and see if that changes. 

John B, not using voice assistant. 

Thanks again, I’ll report back on my haunted Sonos

If that’s the case, certainly @melvimbe‘s suggestion of a diagnostic within 10 minutes of the event, and calling Sonos would certainly help, as they should be able to see what triggered the playback, and tell you.