Possible Clues:

• Presumably, you'd expect to get this leaving The Foggy Dew? Good news after you've been sick!
• Siren at air raid's end
• Signal saying danger has passed
• Danger's over!
• Everything is free and you can't get any better than that
• After the raid it's heard nobody found guilty
• Everyone easily understood signal that danger is over
• There's no danger, as everyone understands ..
• No danger of fog, it's understood
• Permission to proceed once obstacles have been removed