How do I access the secret challenge?

heatherb

I collected the three treasure chests and got the message that I unlocked the secret challenge. How do I access the secret challenge? Clicking on the info trophy shows me how much time I have left to unlock the secret challenge, but literally there is nothing to indicate that I already unlocked it.

Randle

If I could I would reveal the secret heatherb.

Were you able to get it in the end? What was it like?