Related Clues:

• Like sentries
• Like an excellent guard dog
• On top of things
• Not drowsy
• Word with red or on
• Observant
• Red state?
• Having heightened senses
• APB, for one
• A serious one may be red
• Storm warning
• Emulate Paul Revere
• "Fore!" for one
• Give a heads-up to
• Readiness condition