The secret room leads to a secret encounter; to meet this secret character, you should talk to everybody in the village and found out an unsolved mystery :D
I found the secret room*SPOILER ALERT* *DO NOT READ IF YOU WANT TO FIND THE SECRET ROOM YOURSELF*
The secret room cannot be found with the phase detector. Steps:
(I don't know if you can do this at anypart of the game, but i did it before i finished the game.)
1) At the entrance of the village, go left twice to the area with the loud plant thing.
2) There should be some trees in the background. The secret room is located in a very small area between those trees.
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Short and good comment, 11/10