In Queer Street by Fergus Hume

That night Hench awoke during the small hours of the morning with the conviction that he knew all about the mystery in which he was involved. He had fallen asleep much exercised in his mind so far as the visit of Madame Alpenny to Cookley Grange was concerned. He remembered that about the time mentioned by Gwen the Hungarian lady had gone away from Bethnal Green, presumably to procure an engagement for Zara in a West End music-hall. Certainly that might have been one very good reason why she had remained absent for a few days, but now it appeared that there was another, which had to do with Madoc Evans. When unconsciousness came Owain was still wrestling with the problem, and somehow it seemed that the same was solved during slumber. But with the working of his physical brain the scheme broke up, and he was only able to retain fragments. These he proceeded to piece together while staring at the ceiling through the faint twilight of the already dawning day.

It was rather a difficult task to put two and two together.