It was half-past seven and the daylight was beginning to wane when the taxi-cab reached the House of Retreat. Without troubling about his companion, the chief detective rushed into the porter's lodge:
"Mrs. Kesselbach's maid has just come in, has she not?"
"Whom do you mean, the maid?"
"Why, Gertrude, one of the two sisters."
"But Gertrude can't have been out, sir. We haven't seen her go out."
"Still some one has just come in."
"No, sir, we haven't opened the door to anybody since—let me see—six o'clock this evening."
"Is there no other way out than this gate?"
"No. The walls surround the estate on every side and they are very high. . . ."
"Mrs. Kesselbach, we will go to your house, please."