You are turning more stuff off than I do, but that is okay, you are in deep six land and need to know what is going on.
I have a Widget that I use called Active Applications. It shows me all apps that have not terminated. I found it quite useful, and indeed today for some reason, Google Music Player started running w/o me know it. I did not start it...
I immediately knew something was running because after work, I looked at the phone and my battery was way down at about 82%.... very unlike a typical day when it should have been 93% or more.
I went straight to the Active Applications Widget and it showed me Google Music Player. I used Active apps to kill the music player and all is well.
I think this widget is a part of the Samsung utilities that came with the phone.
If you can't find it in your Widgets list, then download this app and see if it shows you anything
https://play.google.com/store/apps/details?id=com.elnware.ActiveAppsAds