Usually it is located in a random room in zone 4.

In this case I was using the active item "Water bucket".

If you use Faily you can see all secret rooms in a level/zone.
In the level with Corny's secret room you will see a secret room with a skull.



If you want to unlock Faily you can check my other guide:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2435653360

This character is available only if you have Rogue Glitch Ultra (so not the old Rogue Glitch).