Question: Why did they feel the need to install red lights and sirens on each of the Dorothy devices? Were those just to alert the operators when the machine was activated?
Question: During the hilltop tornado scene, Preacher mentions "the cone of silence." What exactly is that?
Answer: In the context of the movie, I believe they're referring to a moment where it appears everything is over, but is about to start up again. In real life, the term is evidently a term for a specific area that weather radars can't necessarily pick... usually very close to the radar since being overhead would mean they're outside of the radar's angle.
Answer: In the TV series "Get Smart", there was a device called the "cone of silence." It's used so that no one could hear private conversations, but it never worked properly. It was most likely a cultural reference.
Answer: Exactly this. How do you know your oven is switched on? Because it has a light on it.