18 slam he opened the latch of No.

17 so cautiously that no sound was forthcoming.

Soon he had an ear to Theydon's letter box and was following attentively a one-sided conversation.
Now, Theydon had thought hard during the few strides from one flat to the other.

His telephone was fixed close to the party wall dividing the two sets of apartments and he was not certain that, in the absolute quietude prevailing in Innesmore Mansions at that late hour, a voice could not be overheard.

True, he did not count on Furneaux playing the eavesdropper at the slit of the letter box, but he resolved to take no risks and say nothing that any one could make capital of.
So, when he had asked the exchange to reconnect him with the caller who had just rung up, and he was put through, this is what Furneaux heard: "That you, Mr.Forbes.Sorry I sent my man just now with a message that must leave sounded rather curt, but the Scotland Yard people kindly excused me, so I can give you a minute or two....