Related Clues:

• Qui vive
• "Hey, you!"
• All ears
• Furtive whisper
• [Look over here!]
• Wide-awake
• Ready to jump, perhaps
• Sotto voce call
• Carol opener
• On guard
• Subtler alternative to 'Yo!'
• "Hey you!"
• Wary
• 'Hey, buddy'
• Whistle, maybe