Related Clues:

• 'Look out below!,' e.g.
• Awake and aware
• Keen-eyed
• Like watchmen
• Paying attention
• Like sentries
• Like an excellent guard dog
• On top of things
• Not drowsy
• Word with red or on
• Observant
• Red state?
• Like a worthy watchdog
• Having heightened senses
• APB, for one