There’s a few possibilities. One would be an alarm set up in the Sonos controller, which could be fairly easy to check.

If you also use Spotify, you could have allowed someone access to your Sonos system while they were at your home. Spotify Connect maintains a connection to your Sonos system until you choose a different stream, not a Spotify stream. So it’s possible a friend is trying to play something locally, without realizing they’re still connected to your Sonos via Spotify Connect.

However, if you were to submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it, or post the number here, they would likely be able to help you identify where the ‘play’ command came from.

Thanks.  I will have to do that.  I have owned the system for two years and it just started.  No one else has had access to my Spotify, so that is not the issue.  

Thanks, again

It’s not your Spotify that they need access to. They just needed to be on your WiFi at some point, and have played to your Sonos speakers using their Spotify via Spotify Connect. That connection isn’t broken when they leave your local wifi, or if you use your own Spotify, it is only broken when you stream something other than Spotify on your Sonos speakers. 

Interesting and thanks - the scenario that you mention has never occurred.

Then go with the diagnostic I suggested, within 10 minutes of the speakers playing at 3:30 AM, or check your alarms, and any other home control applications you may have.