About 10 minutes ago, I heard a big boom. Then another, and another. As I was standing in the dining room assessing what direction the sound was coming from, I caught a glimpse of something bright through the windows through the atrium and garage, so I went out front and realized that the noise was fireworks. Honest-to-goodness real Fourth-of-July style fireworks. Coming from the neighborhood on the other side of the street. They were just high enough that I could see them through and over the shorter redwood trees across the street. Bizarre. Peculiar. I stood in the driveway watching for about 10 minutes until they were all over. Sort of cheery for a quiet Friday night.
Best guess: something going on over at the high school. I can't believe it was coming out of someone's back yard, though that's what it looked like from my driveway.