What is the unwanted music source? 

Believe it’s spotify, but not through spotify connect.  

Hi @C0rowe, welcome to the Sonos Community!

I’ve had a look into the diagnostics you’ve sent and the last playback was triggered by the Android device you sent the diagnostics in with.

You are correct that the playback was with Spotify but it does look like it was via Spotify Connect. When playing to Sonos speakers with Spotify Connect, it will stay connected until the session ends. While you may not have initiated playback through Spotify Connect, turning off your Bluetooth headphones will have caused your phone to re-initiate the Spotify Direct connection and start the system up again. 

This is the same on other third party equipment and is due to how Spotify registers devices to work with Direct Control. The best way to de-register the devices to prevent this issue, would be with one of the following steps:

• Start playback to your Sonos System from another music source, e.g. your music library, another music service, radio, etc.
• Trigger a Sonos alarm on your speakers.
• Group or Un-Group speakers that were using Spotify previously.
• Go into the Sonos App, tap on the now playing widget at the bottom of the app and select “End Session”.

I hope this information helps!

Thanks very much!