From the looks of things, it's a common water stain. I tumble my brass in a mason jar with some dawn and citric Acid. I eyeball both quantities so the results are a little variable. The water spots are a common result because I loosely wipe the brass down with a towel and let them air dry. The less water that's left on the brass, usually the more shiny and spotless they are.
Take extra time to make sure your brass is as dry as possible when you put it in the oven. Little spots are NOT going to affect or harm your brass' lifespan as far as I'm aware. If you're really not liking the spots, I'd just clean the brass again.