While this is happening, have you submitted a system diagnostic , or at least within 10 minutes, and called Sonos Support to discuss it?

If I were to guess, it’s likely a “stuck” button, on either the Arc, or a device that is running the controller. There should be information in that diagnostic that Sonos can read and tell you where the volume command is coming from. 

Thanks. How do I go about submitting a system diagnostic?

Click on the underlined words “system diagnostic” in my post. It will take you to an FAQ that explains how to do so. 

Edit: Apologies...you’re most welcome :)

I have a further thought to consider - if you turn off the touch controls on the Arc in the Sonos App ‘room’ settings, does that resolve the issue? If so, and the TV is perhaps very close to the Arc, maybe try moving the Arc slightly further away from the TV - I’m thinking the controls might be being affected by some kind of electro-magnetic field emitting from the TV (perhaps ?)

Thanks Ken_Griffiths

I'll give it try but don't think that is the issue as I've had this set up for about a year with no issues. It only recently started to happen. It happens infrequently.