If you are talking about the one with the loop symbol, then there is a hidden button on the second room in A2. There's one last mushroom minigame there which is a nightmare to complete so I'll just post the solution here if you don't want to do it right away (Although you will have to, cause there's another hidden room to get in A1 that leads to a treasure that can only be unlocked by succeed the mushroom challenge)
Loop symbole is : Green - Pink - Cyan - Pink - Cyan