On crowded weekends like this, unfortunately, there are lots of emergencies and lots of sirens. WMF doesn't post them all on Facebook, but I do try and tweet them out on the WMF Twitter feed. So if there are sirens and you're wondering why, best to check either the WMF Twitter feed or the WMF web site (where the twitter feed is also).
Those recent sirens are an accident in Marshall:https://twitter.com/WestMarinFeed/status/1317582886082543616?s=20
WMF Web site:http://www.thewestmarinfeed.com
You can also download the PulsePoint app which will give you the same information in real time.