When instructed to search a non-public zone (hand or library) players can ‘fail to find’ what they are looking for if the card that is being searched for has a specific characteristic. This most often applies when searching through a library. For example, when a player resolves Squadron Hawk‘s enters the battlefield ability he can choose to not find additional copies of Squadron Hawk even if there are copies in this library. This is because the player doing the searching cannot prove that there is not a card of the given quality without also revealing the entire library. (Keep in mind that when performing a search, even if you fail to find you must perform the other actions like shuffling the library. This is true even if search is not really performed.)
However if a player is instructed to find a card, and there are not additional qualities of characteristics given, then this player must find one. This is because a player can show that they have retrieved a card without revealing what the card is or any other cards in the searched zone. For example, when resolving Diabolic Tutor, its controller will have to find a card and cannot ‘fail to find.’
‘Failing to Find’ does not apply when instructed to choose a card from a revealed hand. Choosing an object is different from searching for one. For example, Duress instructs its controller to choose a non-creature card from the revealed hand of the targeted player. The controller of Duress MUST choose a non-creature card if there is one in this player’s hand, and that card will be discarded.