Well we all know that Snape is not concerned nor really cares what Harry sees that because in Snape's mind, Harry is there to learn to close his mind. He thinks Harry is weak, he probably thought he would never retrieve his worse either. As far as the one he is hiding, I guess Snape assumed to be better safe than sorry. It all just makes us wonder besides the obvious could he have been hiding anything else we may not know of yet.