What are key wardens? Randomly hidden boss fights that require rift keys? Are they in the normal boss fight areas?
They're scattered throughout an area in each act.
They drop a specific key needed to craft an infernal machine.
Key of Bones - Act I, Fields of Misery
Key of Gluttony - Act II, Dahlgur Oasis
Key of War - Act III, Stonefort
Key of Evil - Act IV, Silver Spire Level 1