Code blue
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
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 the code for if "you saw a Cylon and you knew it."