Severus had terrorized House Elves and ghosts alike in his search for Potter. He would demand that his memories be returned, then he would Obliviate him to ensure he never thought of them again.
He finally found Potter leaning against a wall, completely unaware of his surroundings. Should he chose, Severus could have Obliviated him right then and there. But first, he needed to find out if his memories were still in a Pensieve, or in the boy's possession.