I had a ruling on this by a L2 Judge at a TCG event in San Diego 2 weeks ago. I believe his ruling was wrong.
I played Praetor's Grasp
Mana Cost: 1BB
Card Text: Search target opponent's library for a card and exile it face down. Then that player shuffles his or her library. You may look at and play that card for as long as it remains exiled.
I searched the opponent's library and pulled a Snapcaster Mage. The opponent asked if he could look through his library to determine what card was taken. I told him he could not, card doesn't say so and it violates the spirit of the card. The head judge was called and ruling was issued that a shuffle takes place in a hidden zone and that while cards are in a hidden zone they may be looked at by the owner as long as it doesn't take too much time.
I believe that the RTFC rules don't say anything about the opponent searching his deck.