One thing that comes as a surprise to many people is that the secret missions are *only* in Expeditions modules, not in Encounters or in the published adventures. So if you leveled a character up to Tier 3 via Rise of Tiamat or Princes of the Apocalypse, you've essentially leveled past *most* of the current content which had the secret missions. As Skerrit pointed out, they will show up in Tier 3 content, but as there's been only 1-2 secret missions for each faction per season...it's probably best to know that we're not going to see that pace drastically increase.