This area has 1 Secret Rune that can be collected only after completing the main campaign. Check the video at the end of the page if the written walkthrough is not clear.
Secret 1 (Secret Cow Level Rune)
You’ll find it at the end of the map. Right side of the exit gate there is a pressure plate hidden among mushrooms. Press the plate to open a secret passage. Go inside and collect a Secret Rune. Collecting 9 Runes will unlock a Secret Cow Mission.