Couple of things.

First, Sonos speakers play even when there are no controllers on your WiFi. You can start playback from a stream, then leave the house with your phone, and the Sonos speakers will still continue to play that stream. That why the controller is merely a remote control, the actual app is running on the computer in the Sonos devices.

Second, I’m guessing you have Spotify Connect, and have had guests over recently who have used it to play music on your Sonos. Once they’ve connected to your Sonos while on your WiFi, they remain connected to your Sonos even when they have left your WiFi. So, they are likely at home, bring up the Spotify app, and forget that the last ‘Connect’ process was to your Sonos. They try to play something, and not being in your home, they can’t hear it, so they turn up Spotify to see why they can’t ‘hear’ what they are playing.

All is not lost, however. You can, if you’re aware of it, break their connection to your Sonos system when they leave your home, by playing any other source, just once, on your system. That ‘breaks’ the connection they have via Spotify Connect, and they can’t send music to your system until they return to your local WiFi signal.