The line comes from Warhammer 40,000: Dawn of War, the 2004 real time strategy game by Relic Entertainment, where Imperial Guardsmen units announce contacts with this response. In Warhammer 40,000 lore the Guardsmen are the ordinary human soldiers of the Astra Militarum, known for massed infantry and desperate holds against far stronger foes. Clips of their voice lines spread through gaming forums and YouTube edits because the delivery is so earnest and panicked. The enemy spotted call remains popular as a reaction for any sudden threat reveal.