Dominion Herald's card is not an action, then what?
The card is put back on top.

From the original rule book:


  “Reveal” – when a player reveals a card, he shows a card to all players and
  then returns it to wherever it came from (unless instructed specifically to
  put it elsewhere). If the player is required to reveal cards from the top of
  his Deck, and he does not have enough cards, he shuffles in order to
  reveal the required number of cards.