I am using spotify connect functionality a lot. 10 hours and above of continuous playing is nothing special. I don't know if this particular information is useful, but the "wi-fi speaker" I am using is Yamaha R-N500 amplifier - Spotify integration on this device really sucks by the way - auth errors, playback stop after each song, music not playing on first try, etc. - you should be ashamed guys.
Anyway, I'm igniting the music playback via PC or iPhone. Recently I've noticed that if I play the music by the phone, then it's battery will drain soon (even though it actually does not play music, it actually does nothing in such scenario).
I have done a little research and I'am pretty sure that it is caused by the integration of Spotify app and iOS's volume control mechanism. Few versions ago, when music was playing by spotify connect and the application was running in background, pressing volume buttons would not cause change in the volume of external device (the one playing music). Now it does, and since this feature has been introduced battery drains.
Battery does not drain, when I play the music and stop the application so it does not run in the background (obviously). Most importantly, if the application is running and connected to spotify connect device, both phone speakers are constantly turned on. To be more specific, their amplifiers are constantly turned on which can be recognized by audible hissing heard in quiet environment.
Last point is my favourite. The hissing sound is present both if the phone is active and when the screen is locked and dimmed. This behavior is reproducible.