Music playing on iPhone unknown source. No apps open

Hello lemmontage,

Thanks for visiting the Apple Support Communities. We're happy to help checking music playing.

Some apps can play in the background, and just because an app is not on open and active, does not mean it's not open.

To check, open the App Switcher: Switch apps on your iPhone, iPad, or iPod touch - Apple Support

Here, you will see all open apps. Swipe up one by one to force them to close. See which one was playing music, so you know what to check for next time.

Thanks.