It's showing up on the very top of the home screen, the speed dial section. There is a small selection of songs that are shown there and it's the first place I go looking when I'm wanting to play music, so something that isn't usually there definitely catches my eye.
It would be a VERY strange coincidence for sure. I think I'll test to see if I can make it happen again.
Anyways, I guess is there any reason why I should believe that any app under Graphene OS could possibly use the microphone without explicit permission or without displaying the little green dot in the top corner of the screen?