Its almost always Spotify.

If you really want to know, call Sonos. 

Super weird - I have Sonos at my airbnb, and my guest was woken up at 4.23 ET this morning by loud music. This has never happened before.

Trying to figure out if it’s connected to a previous guest’s phone, but it’s my understanding that once you’re out of Wifi, you can’t control the system.

Except for Spotify Connect, that’s true.
 

In order to ‘break’ a Spotify Connect, well, ‘connection’, the Sonos user must play another streaming source.

But there are also alarms that can be set up using the Sonos controller, as well. It’s hard to be definitive about what caused this, without having Sonos look at a diagnostic that was generated within 10 minutes of the occurrence.

I’d be putting money on the Spotify Connect possibility. The person generating that signal can’t ‘hear what they’re playing, not realizing they are still connected to that Sonos speaker, and will turn up the volume, under the assumption that they mistakenly turned it down.