You're in the middle of the war, watching an enemy camp. They every day bustle that would fill such a camp is there. From all appearances and sounds, there is no doubt in your mind that unit is still there and reports of them moving must just be a mistake. Only, it turns out that you were the one mistaken. Your eyes and ears were deceived by the 23rd Headquarter Troops, or the ghost army, and their theater of war.
This troop of artists was the subject of yesterday's Stuff You Missed In History Class. They used their amazing creativity to put on "plays" for the enemy, so that their fellow soldiers could move safely to different places and to make the Germans think that there were more troops than there actually were. They used a myriad of techniques to confused the enemy: from inflatable tanks and airplanes, using different outfits and military patches, using recordings and blasting them at the enemies and even going out for omelets just to talk a little too loudly about military type secrets.
They're credited with saving quite a few lives and helping to secure victories. Their story was classified until 1996, making this a not super well known story. It's really interesting stuff, so I suggest listening to the podcast. If you're more into documentaries and have Netflix (US at the least) there is a documentary called The Ghost Army. It has interviews from men who were part of the unit and other information. (Or you could do what I'm doing, and watching Netflix after listening to the podcast.)