Secret Hunter is a pretty old archetype and had been one of the more popular decks for the Hunter class. It was largely enabled by Cloaked Huntress, which was released in the One Night in Karazhan adventure. This allowed you to put multiple Secrets down in one turn which could gain you a lot of tempo and buff Secretkeeper quite a bit if you had one on the board. Once Huntress rotated out of Standard, the deck was then put aside.
Enter The Boomsday Project and the card Subject 9. You can now draw 5 different Secrets from your deck on turn five! That’s some pretty heavy draw power, and combined with some other strong Hunter cards, the deck is starting to gain traction.