Is there anyone else on your wifi network? Or a Spotify user that might still have access to your system? I can’t see any reason the volume would change other than because someone has decided to change it. 

There is nobody else on my secure home network. 
It’s a pain, as I have to keep an eye on the volume just in case it goes up. Bear in mind it’s not during play. It’s only when one side is finished, and I flip it over and even thought the volume is set at 50%, it sometimes just goes way up?

I’ve certainly never seen Sonos to behave in this way. I think the next time it happens, you should submit a system diagnostic within 10 minutes, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

​@Cr1st0f001 

Odd behaviour indeed. But if it helps, you can set a volume limit in “Settings - gspeaker/room name]” so the speaker can’t go above the volume you set. 

That’ll sort it. Cheers. My neighbours will be pleased  

You could make a diagnostic just after this happens and call Sonos about it.