I'm guessing the second minion on your opponent's board was Spirit of the Frog? If so, when your opponent played Haunting Visions, Spirit of the Frog added a 4-mana spell to his hand (maybe that Hex) before the discover. That meant his hand was full and the discovered Ancestral Spirit went to his graveyard instead of his hand. Cards in the graveyard are visible to both players, so it was revealed in your history.
Josh Durica
I'm guessing the second minion on your opponent's board was Spirit of the Frog? If so, when your opponent played Haunting Visions, Spirit of the Frog added a 4-mana spell to his hand (maybe that Hex) before the discover. That meant his hand was full and the discovered Ancestral Spirit went to his graveyard instead of his hand. Cards in the graveyard are visible to both players, so it was revealed in your history.