I thought I had left the Spotify app running when I shutdown the Mac. It turns out that Spotify set itself up to start up after login.
The more interesting thing is that you can’t disable this automatic startup from the Spotify app.
The method to stop Spotify from starting up after login, is to:
- Go to > System Preferences > Users and Groups > Login Items
- Select Spotify
and click on the 
(minus) icon.