Spotify Connect (Sonos) while not on home Wifi still "steals" connection

[arwolk]

I've recently noticed that my Spotify Connect devices (Sonos) are available within my Windows desktop Spotify app, even while not at home and connected to the wifi where those devices live. I can even choose to switch the output to my home Sonos instead of my laptop, and it actually works - which really freaked out my girlfriend one day when the music started blasting!

Now, while at work using my Windows desktop Spotify app, sometimes she will try playing music on the home Sonos (from the Sonos spotify app) and it will "steal" my connection. I can see in my desktop app that the music has started playing on my home devices. When I switch back to my laptop, it stops playing at home.

How can I fix this problem? Shouldn't Spotify Connect only work while connected to my home wifi?