You should not be able to see Sonos at all unless on the same network.  If you can, then you are probably exploiting the ‘feature’ of Spotify Connect that allows a cloud based link between the Spotify app and a Spotify Connect enabled speaker to persist after the controller device has left a wifi network.

It is very questionable whether this is desirable at all.  Why do you want to do this?

If this is a correct interpretation then only the Spotify account most recently connected to the wifi would be able to see the Sonos speakers. In my view that is one too many, not one too few.

Thanks for the answer, @John B , somehow I didn’t see you replied.

Yes, I was referring to this Spotify Connect “feature”. To me, it’s a nice feature, for many things: I have two networks at home (one “home”, one for work, and I don’t want them to be connected, for security purposes), and I’m forced to switch to the Sonos one to change things. Moreover, I can stop music in case we leave home and it was turned on, and other uses that are not critical, but very useful.

If the behavior is the one you say, then there’s something not working the way it should, since on of the spotify accounts sees permanently my Sonos, even if it hasn’t been connected to the wifi for some days, while the other one only sees the Sonos on Wifi.

Weird, isn’t it?

Yes it is odd, but I don't know for sure if it is supposed to work the way I stated - it just fitted my understanding of how Spotify Connect 'must' work.