Between the time of his first examination of the pavilion and the night of the murder of the keeper, Larsan had had time to find out what those papers contained.
He could do nothing with them, and they were rather compromising.
That night he took them back to the chateau.

Perhaps he hoped that, by returning the papers he might obtain some gratitude from Mademoiselle Stangerson.

But whatever may have been his reasons, he took the papers back and so rid himself of an encumbrance." Rouletabille coughed.

It was evident to me that he was embarrassed.
He had arrived at a point where he had to keep back his knowledge of Larsan's true motive.

The explanation he had given had evidently been unsatisfactory.