In the Mission I could occasionally hear foghorns, which made no sense to me. It was generally around first light in the morning. After quite a lot of digging, including making heatmaps of where ships parked, I traced it to Anchorage No 9, which is basically a parking lot for giant ships. You can see it when crossing the Bay Bridge.

More details and maps here: https://www.facebook.com/williamp/posts/10106726609193073