Around the map there are pictures with Nova-6 gas coming from behind: one at Beds, one at Lounge and one at the 2nd floor of the Green House. You need to guide a 'brain rot zombie' near the pictures to knock them down. Such zombie can be created by using the alternative ammo weapon upgrade of the Pack-a-Punch, or by using the CTRL Z Elixir.
Behind the pictures, you will find three codes that will need to be entered into Rushmore.