What should you do if you hear a fog signal of another boat you cannot see?

If you hear a fog signal from another boat that you cannot see, you should first reduce your speed to a safe level and maintain a proper lookout. Use radar and other navigational aids to locate the other vessel if available. Be prepared to take evasive action if necessary, and consider sounding your own fog signal to alert nearby vessels of your presence. Always prioritize safety and navigation rules in low visibility conditions.