And just consider, gentlemen: if Lupin had come last night, and if he had really set his heart on the coronet, he would have stolen it then, or at any rate he would have tried to open the safe in M.Gournay-Martin's bedroom, in which the coronet actually is, or this safe here"-- he went to the safe and rapped on the door of it--"in which is the second key." "That's quite clear," said the inspector.
"If, then, he did not make the attempt last night, when he had a clear field--when the house was empty--he certainly will not make the attempt now when we are warned, when the police are on the spot, and the house is surrounded.

The idea is childish, gentlemen"-- he leaned against the door of the safe--"absolutely childish, but Guerchard is mad on this point; and I foresee that his madness is going to hamper us in the most idiotic way." He suddenly pitched forward into the middle of the room, as the door of the safe opened with a jerk, and Guerchard shot out of it.
"What the devil!" cried M.Formery, gaping at him.
"You'd be surprised how clearly you hear everything in these safes--you'd think they were too thick," said Guerchard, in his gentle, husky voice.
"How on earth did you get into it ?" cried M.Formery.
"Getting in was easy enough.

It's the getting out that was awkward.
These jokers had fixed up some kind of a spring so that I nearly shot out with the door," said Guerchard, rubbing his elbow.
"But how did you get into it?
How the deuce DID you get into it ?" cried M.Formery.
"Through the little cabinet into which that door behind the safe opens.
There's no longer any back to the safe; they've cut it clean out of it--a very neat piece of work.

Safes like this should always be fixed against a wall, not stuck in front of a door.

The backs of them are always the weak point." "And the key?
The key of the safe upstairs, in my bedroom, where the coronet is--is the key there ?" cried M.Gournay-Martin.
Guerchard went back into the empty safe, and groped about in it.