In all the various threads that have reported similar occurrences over the years, it’s been one of two issues.

Most commonly, it’s been someone who created a Spotify Connect connection while in your home, trying to play a stream in their local environment, and not realizing that the Spotify Connect connection is still active.  Because they’re not hearing anything, they tend to turn things up before realizing, if they ever do, that it is playing elsewhere. However, as the Sonos owner, it is relatively easy to break a Spotify Connect connection to your Sonos. Once the people who have had access to your system have left your home, simply play any other source that is not Spotify on your Sonos. 

The other, less common issue, has been some sort of alarm from a home automation system, potentially HomeKit, which was set up once and then forgotten. 

So far, none of the resolved and reported cases in this forum have been an issue in the Sonos software itself has been responsible, but as that software is constantly changing, it’s not impossible. However, in order for the Sonos folks to help figure out the source that is causing the issue, you would need to submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it, or post the diagnostic number here for a Community Moderator to pick up.