Spotted an old air raid siren in Eagle Rock the other day which prompted some research. They have them in SF too and when I lived there in the 90’s I remember I would wake up when they tested them on Thursday mornings. Not sure if they still do.
The sirens were installed during WWII all over Los Angeles. They often short circuited and went off causing widespread freak outs. During the infamous “Battle of Los Angeles” when unidentified objects were spotted flying over the ocean, 5 people died during the black out that ensued - from car accidents and heart attacks.
In 1985 the sirens were turned off permanently and the ones in the worst condition were torn down. But there’s still plenty around. You can see some pictures here.