Well, I think if they had a secret idol, they would've at least shown us a silhouette or something. Unless the secret idol is Kakusei Miki XD
They could still be waiting to release something big...I don't know, I almost feel like they should've released more stuff, too. We're usually sitting on the same information for a long time before they pop up with something else.
I'm sure someone'll go iM@S2 crazy and figure out the secret achievements not long after 2/24.
Maybe one of them is surviving Hell Training? XDD