Code blue
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The precise meaning of this phrase is not known, but Chief Tyrol forcefully said it shortly after he recognized Cavil as a Cylon agent and tackled him. (Lay Down Your Burdens, Part II). Presumptively, it designates some sort of emergency, and would be applicable to the discovery of a previously unknown Cylon agent. The podcast for Lay Down Your Burdens, Part II says it is what someone would be shouting "if you saw a Cylon and you knew it."