Do you have Spotify?  If so, someone may have been on your network controlling Spotify via Spotify Connect, and now they can access remotely.  The cure is to play another source to the rooms where this is occurring.  This will sever the connection.  Then, don’t allow people to connect to your Wi-Fi LAN.  Consider setting up a guest network. 

Actually, no one in our house uses Spotify.  We typically only use Pandora.  Sometime we use the Sonos Radio station.   It is Pandora that is playing each time it happens.

This is the first time I’ve heard of this being an issue with Pandora. Since they’re out in the lanai, any chance there might be an animal jumping on them, triggering the ‘play’ and volume? There have been some reports that suggest sometimes those buttons are sensitive enough to be triggered by dust ( and I’d assume some extra nudging, but the issue went away when the device was wiped down). I’d be tempted to do two things. First, go in to the room settings in the controller, and turn off the extra hard buttons. Then, if it still happens,  submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

FWIW, the slight heat generated by a Sonos speaker is extremely appealing for all sorts of nocturnal creatures. Mine was a cat, but others have discussed lizards and frogs as potentials, too. 

I did find lizard/frog droppings on the ground under one of the speakers but I didn’t think much of it since I had turned off the hard button controls.  I’ll clean the tops and see if I can find a way to block any critters from resting on them, and see if that works.  Thanks.

Shouldn’t be an animal, if you’d previously turned off the buttons, that’s why that diagnostic to Sonos is so important. 

If it was Spotify, I’d suggest it was a Spotify Connect issue, but since you say Pandora, I’m interested in what Sonos finds in the diagnostic.