Ok... I just read that it is a Treasure Hunt. The reward is a bunch of gold bars:
If someone solved the hunt, please post a guide here.
I attended the New Year's countdown event in Japan Central Plaza. Seem to be one of the only english users who did.
I can confirm there was no reward there either. There was a nice 360 degree firework display in the sky with techno music, though. Sounds like it was slightly more exciting than the Granzella events.