Mystery source of songs playing on Homepod

I play Emo music continually for my cats via a single playlist on my Music app, named Lullaby. My iTunes is 100% podcasts.

Occasionally I return home and find the music has stopped. I tell Siri to play my Lullaby playlist on the HomePod and play resumes.

In the morning, I usually turn the volume up a bit for the cats to enjoy. Sometimes I notice a new song that I like but don't recognize, so I'll check What's Playing, and that's where the mystery begins.

I open Music, but its status is "Not Playing". I open Playlists anyway (in disbelief), but sure enough, Lullaby is unopened and the status remains "Not Playing", which is why I'm really confused.

My understanding is that the only source options for the HomePod are Music or iTunes.

If Music isn't playing and I have no music on iTunes, then from where are the songs being selected?

None of my efforts to find HomePod play history have succeeded.

I'll be grateful for any help y'all can offer.