Shhhh, it's a secret

Intelligence experts in the United States are trusted with this nation's secrets. Within the hidden walls they perform their work, but upon emerging from their lairs they cannot speak of what they have done or even what classification level they have been working on. In order to talk "freely" a not-so-secret cover term has been used informally when these operatives where in public. Instead of saying, "I can't talk about that here it's Top Secret Umbra" or something similar they would say, "I can't talk about that here it's Secret Squirrel".

-Professor Walter