I heard about it on Serebii.
As you play through the demo multiple times, there is a small chance that you will be able to activate the secret mission. This mission has you go to an island where there are no wild Pokémon. On this island, you will encounter Team Aqua or Team Magma and have to team up with May. The team you face is random. As you battle through the grunts in the demo, teamed with May & her Raichu and Swellow, you will learn that they are there to hunt for the Mega Evolved Pokémon still. Upon completion, you will anger Matt or Tabitha depending on the team and they'll report back to their leaders. This special mission doesn't provide any new items, just new battling opportunities. After you have completed it, it can be selected again at any time from Steven's options, by selecting the phrase "Let's Go Somewhere Secret". -Serebii
I don't quite understand what you want to know here, but this pretty much summarizes everything.
