Its obvious Zone is making an Engie Hentai flash to celebrate the upcoming engie game. Or All the girls EXCEPT Engie are going to get fucked while Engie laughs evily. Either way its a secret because Engie fans be crazy (Engie 4 Life)
The big hint here is Zone not wanting anyone to be told about it, she's pretty overt about sexual stuff so it's obviously something like flower-picking or binge-watching Lost or something.