Possible Clues:

• Siren sound to mark end of bombing by planes
• No-one doubts what this signal means
• Permission to proceed, with no further questions needed
• Siren sound to signal end of air raid
• "Danger has passed" signal
• Doctor¿s opinion signals end of battle
• Danger-has-passed signal
• 'It's safe now'
• Signal that it's safe to come out
• 'Danger over' signal