If you make a diagnose within 15 minutes after this happens, Sonos will know (when you contact them) which phone is the culprit..

While a a system diagnostic would certainly help Sonos figure out where the command is coming from, your use of the phrase ‘from Spotify’ is a good indication that the command is coming from Spotify Connect, from someone you have previously had in your home and gave access to your Sonos system to play music. You likely need to ‘break’ that connection on your Sonos devices, by playing something other than Spotify for a moment. That kills all outside sources from being able to connect to your system, and you can then go back to using Spotify without fear of someone else being able to use Spotify Connect to play to your system. At least until the next time they’re in your WiFi, and can reconnect to your Sonos, after which doing the same thing, playing a source from anything other than Spotify will break the Spotify Connect links again.