Possible Clues:

• Engineers by valley having radio telegraphy to give warning
• Emergency on the Enterprise
• Signal of imminent danger
• Warning to heed
• Communist later beaten up in heightened security state
• Scarlet warning of imminent danger?
• Siren's signal right, altered wrongly
• State of readiness for imminent threat
• 'The enemy is upon us!'
• Pre-meltdown signal, maybe