"In this mission, you're back in Steven's store, and you have to kill 20 zombies. As the title indicates, the zombies are all invisible. They still make noise, have shadows, and you can still see them slightly, but it can be tough to spot them despite this. The best thing I can tell you is to memorize their locations, and kill them quickly (or, obviously, just watch the video). It's best if you kill them from behind, as they won't try to swing at you, and it will save you time. Do this in about less than two minutes, and you'll get the S rank."
The zombies have dull shadows below them, making it slightly easier to detect them.
The zombies are actually slightly visible when close by.