The Secret of the Caves by Franklin W. Dixon

Vacation time came, as it always does, although the days dragged, and when the last examination was written and the Hardy boys and their chums faced the long summer holidays, the boys had more exciting concerns than the affair of Todham Todd.

Miss Todd had left the Hardy home, after profuse thanks for the hospitality the family had shown her, and had returned to the college town. Mr. Hardy, after spending a day or so there, had gone on to parts unknown and it was assumed that he was following clues that he hoped would lead to the discovery of the missing professor.