A person is revealed to be a spy by speaking in a foreign language or accent
A spy, saboteur or the like is observed speaking in accent that he was previously shown not to have. Alternatively he could be speaking in a foreign language, or both. This character has let his guard down, probably preforming minimal or no counter-surveillance measures prior to doing so. The character in question thinks that only members of his intelligence agency are around. The things said in conversation are often easily damaging to his cover as well. Contrast Spy Speak, which is making a conversation that has covert purposes seem mundane.