The line comes from Warhammer 40,000: Dawn of War, the 2004 real-time strategy game by Relic, where Imperial Guard infantry automatically announce contacts with specific factions. Eldar are the craftworld Aeldari, fast and elusive aliens who often strike from stealth, so the warning carried real tactical weight in play. Clips of faction callouts later spread through YouTube compilations and soundboards, where this enemy spotted alert became a shorthand for any sudden, unwelcome appearance by a superior foe.