MYSTERY: SOLVED

The mystery bubbles are coming from the roof of Presidio Building 35, which is apparently "The Bay School of San Francisco." I didn't see the bubble-generation apparatus, nor did I see any people, so I suppose it's possible the bubbles may have come from somewhere else and were simply captured by updrafts adjacent to the building, giving them the appearance of having come therefrom, but I think I'm right. I am the world's greatest detective.