The line comes from Meet the Spy, the 2009 Team Fortress 2 short released by Valve to introduce the Spy class. In the scene the BLU team realise an enemy Spy has infiltrated their base and may be disguised as one of them. Clips of the speech spread through gaming forums, YouTube edits and meme soundboards, where the warning became shorthand for suspicion. It remains popular because the delivery escalates so quickly and the idea of a hidden impostor fits games, group chats and everyday paranoia.