He does imply that there IS a secret, along with that he himself doesn't even know what it is. I don't think there's ever been a recorded sighting of the di/mono glyph since the first month of ODST so honestly I'd guess it's been patched or something. QTA may possibly still be at large, but I remember seeing that there was a QTA (quick time application) released on 06/12, hence the VI XII. There's also the random detonator at the bottom of Oni Alpha Site, which we haven't been able to do anything with.
But the fact that it's this late and no one has found anything yet just goes to show one of two things: It's not there, or it's hidden with extreme/excessive work. With the amount of games spent on ODST in Mombasa streets, someone would have found it most likely by chance. So I'd guess it's one of those nonsense things where you have to jump through a bunch of rings in a random order. There's not much we can do about that.