He saw it all--according to his own idea.

Stoner had overheard the conversation between old Kitely and Cotherstone in the private office, of course! That was it--he wondered he had never thought of it before.

Between the partners' private room and the outer office in which Stoner sat, there was a little window in the wall; it had been specially made so that papers could be passed from one room to the other.

And, of course, on that afternoon it had probably been a little way open, as it often was, and Stoner had heard what passed between Cotherstone and his tenant.
Being a deep chap, Stoner had kept the secret to himself until the reward was offered.

Of course, his idea was blackmail--Mallalieu had no doubt about that.