Possible Clues:

• Everyone helps tidy away — a sign of health and safety?
• 'It's safe now!'
• Safety announcement
• Signal that may allay anxiety
• Everybody getting unequivocal signal to come out?
• Sound of siren makes everybody jump
• Call around with real oddball - it suggests the danger has passed
• No one doubts what this signal means
• Tip off Bellow to be sure it's said at the end of raid
• End of air raid makes everyone jump