The shot in the ceiling which the Berniers heard was the accidental shot during the nightmare.
"I had now to explain the wound on the temple.

It was not severe enough to have been made by means of the mutton-bone, and Mademoiselle had not attempted to hide it.

It must have been made during the second phase.

It was to find this out that I went to The Yellow Room, and I obtained my answer there." Rouletabille drew a piece of white folded paper from his pocket, and drew out of it an almost invisible object which he held between his thumb and forefinger.
"This, Monsieur President," he said, "is a hair--a blond hair stained with blood;--it is a hair from the head of Mademoiselle Stangerson.

I found it sticking to one of the corners of the overturned table.