As they threaded the steep side streets already powdered with silver, Angus finished his story; and by the time they reached the crescent with the towering flats, he had leisure to turn his attention to the four sentinels. The chestnut seller, both before and after receiving a sovereign, swore stubbornly that he had watched the door and seen no visitor enter. The policeman was even more emphatic. He said he had had experience of crooks of all kinds, in top hats and in rags; he wasn’t so green as to expect suspicious characters to look suspicious; he looked out for anybody, and, so help him, there had been nobody. And when all three men gathered round the gilded commissionaire, who still stood smiling astride of the porch, the verdict was more final still.
“I’ve got a right to ask any man, duke or dustman, what he wants in these flats,” said the genial and gold-laced giant, “and I’ll swear there’s been nobody to ask since this gentleman went away.”
The unimportant Father Brown, who stood back, looking modestly at the pavement, here ventured to say meekly, “Has nobody been up and down stairs, then, since the snow began to fall? It began while we were all round at Flambeau’s.”
“Nobody’s been in here, sir, you can take it from me,” said the official, with beaming authority.
“Then I wonder what that is?” said the priest, and stared at the ground blankly like a fish.
The others all looked down also; and Flambeau used a fierce exclamation and a French gesture. For it was unquestionably true that down the middle of the entrance guarded by the man in gold lace, actually between the arrogant, stretched legs of that colossus, ran a stringy pattern of grey footprints stamped upon the white snow.
“God!” cried Angus involuntarily, “the Invisible Man!”
Without another word he turned and dashed up the stairs, with Flambeau following; but Father Brown still stood looking about him in the snow-clad street as if he had lost interest in his query.
Flambeau was plainly in a mood to break down the door with his big shoulders; but the Scotchman, with more reason, if less intuition, fumbled about on the frame of the door till he found the invisible button; and the door swung slowly open.
It showed substantially the same serried interior; the hall had grown darker, though it was still struck here and there with the last crimson shafts of sunset, and one or two of the headless machines had been moved from their places for this or that purpose, and stood here and there about the twilit place. The green and red of their coats were all darkened in the dusk; and their likeness to human shapes slightly increased by their very shapelessness. But in the middle of them all, exactly where the paper with the red ink had lain, there lay something that looked like red ink spilt out of its bottle. But it was not red ink.