The Hidden Immunity Idols twist has been implemented in Survivor: The Congo as of yet.
The Hidden Immunity Idol: A Hidden Immunity Idol will be hidden at each camp, able to be found by deciphering clues. The Hidden Immunity Idol can be played before votes are revealed, and will negate all votes casted for that castaway, and the person with the next highest amount of votes will be voted out.
In the first season, Survivor: The Congo, clues to the Hidden Immunity Idol would be given through Independence, for every person that declared independence, everyone would get a clue to their tribe's idol. Also, Idol clues could be won through reward/immunity challenges prizes.
Survivor: The Congo: The HII twist was, while well known in the real world, a welcome change to the game, and impacted the game with 5 idols being played throughout the game, 4 of them being needed. It also caused what was awarded as the Biggest Blindside with Maisy's elimination at the Final 10.