I have a free Spotify account and several devices with Spotify installed. I faced this issue yesterday when trying to listen to Spotify in the office. When I started the application, it said that I was listening to music on my smart TV at home. So far so good, because my girlfriend was listening to music on our tv, but I was able to adjust volume and change the tracks on the TV from the office, despite obviously using a different network.
That made me curious and in the evening, I managed to control the Spotify app on our TV or on my Windows 7 laptop (both were connected to our home network) using my phone, which was connected to a 3G mobile network.
I know that with my account I am only able to listen to music on one device at a time, but this article says that all devices have to be on the same WiFi network the "first time you connect". This might have been the case with my TV, laptop and cell phone but it definitely wasn't with my computer in the office because it uses a different network.
Am I missing anything from the descriptions or is there anything I can do to prevent controlling the app on any of my devices from unknown sources?