Homepod playing music. What is the source and how do i stop it?

I started Apple Music playing an album using my iPad, with a destination of two airplay devices.

When the iPad completed the album, it went on to an infinite play list (fine by me) BUT the iPad did not seem to be the source. There was no 'playing now' on the screen, and the album was showing the triangle to play icon.

... yet the music continued and I want it to stop.

I have Siri disabled (naturally); and the HomePod is out of physical touch. It is a target speaker only.

The airplay speakers were clearly getting the music from somewhere, and it did match the original album played by my iPad. ... only my iPad no longer controls it!!!

How can I determine where the source of the music is?

If the iPad has handed the control off to another device ... how can I stop that from Ever happening again?

... and most importantly ... how do I stop the music playing now ... short of cutting power from the building?!

Thanks.