Since Cerca Trova means "Seek and you shall find" that's the phrase they use if they put hidden images, or links onto the site. For the Witness contest, the paragraph which stated there was a contest I believe ssaid at the very end of the paragraph Cerca Trova, correct me if I'm wrong someone.
Apart from being the forum to discuss "The Cerca Trova forum is a place for intelligent, serious discourse concerning relevant issues and theory in the art of magic" as brilliantly stated by Jack. It refers to hidden things you cannot normally see around the site. And is one of Theory11's "key" phrases besides C+,Nanapancakes, and Danny Garcia is a Ninja haha.
Hope I helped out a bit more.
-RA69