Related Clues:

• Threats from very big blokes inside!
• Warning signs of half the adult population losing head
• Signs nothing -- and people sail without trouble
• Emblems of danger and signals to stop
• Black cats, perhaps
• Foreboding phenomena
• Red skies, to sailors
• Eclipses, to the ancients
• Bad signs
• Signs seen by fictional voyager heading north and south
• Hints of what's to come
• Troubling signs
• Augural observations
• Telling signs
• Signs of things about to happen