If it had been, the concierge and his wife would have heard the noise.

Besides that, it would have been carried down into a main street, in which there are people at all hours.
Somebody would have been sure to tell a policeman that this house was being emptied.

Moreover, the police were continually patrolling the main streets, and, quickly as a man like Lupin would do the job, he could not do it so quickly that a policeman would not have seen it.

No; the furniture was not taken down the stairs or out of the front door.
That narrows the problem still more.

In fact, there is only one mode of egress left." "The chimney!" cried the Duke.
"You've hit it," said Guerchard, with a husky laugh.