Agatha Christie. The visit of a stranger (№ 266137)

- How did you know that the window would be open? - I whispered, because it was just supernatural.
Because I sawed the latch on the window this morning.
What?
- Well, Yes, it was very simple. I called, presented a fictitious card and one of the cards inspector Japp. I said that I was sent here on the recommendation of Scotland Yard to set the alarm system, which Mr. Lavington was asked to mount in his absence. The housekeeper was glad my arrival. It seems that only recently the house has tried twice to Rob... Obviously, our idea came to mind and other victims of this blackmailer, because nothing of value was taken. I examined all the Windows, did what I needed, and the servants were forbidden to touch the window until the next morning, explaining that the frame is connected to the alarm system, and, as if nothing had happened, withdrew.