My Sonos network is spontaneously playing music from Spotify. This happens at random times and on random speakers. For example this morning it was playing on all speakers:

(clicking on a room indicates the Spotify user is “Guest”). The strange thing is no one in our house has a Spotify account or even the Spotify app on any device. The Sonos network does not have Spotify setup as a service. The only way I can see this might happen is if someone were to join the WiFi network and play music from Spotify directly to my Sonos devices.
Our Sonos network is made up entirely of old gear connected by WiFi and we’re using the S1 app.
This happened multiple times this week. It happened previously a while ago (many months).
Other than someone joining the network and playing directly I can only make wild guesses that point the finger at Sonos’ internal systems?
Any ideas on how to figure out what’s going on? Trying to avoid rebuilding my Sonos network; especially if this is something Sonos is interested in diagnosing.
