usually after the release of a game, the same people/magazines who review it have codes and hidden stuff within the same or next few installments. there have been times where some have remained hidden from the public for a few months. would anyone like to guess at which secret within a game took the longest to be found and brought to attention?

Getting Mew legitimately in Pokemon Red and Blue remained hidden for seven years. But then again that's more of a glitch then an easter egg.