Still no clue how. I'm hoping someone here has an idea. I've also contacted the google play music app team.
I don't store any music locally, which seems to make this even weirder since I assume that would be the only time to do anything in the background.
I took a closer look at the logs. Turns out, it was closer to 4 gigs throughout the day. A couple of days worth of music? Insane.