When I questioned him as to how he had employed the time, he told me that he had gone for a stroll in the Bois de Boulogne.

What do you think of a professor who, instead of giving his lecture, obtains a substitute to go for a stroll in the Bois de Boulogne?
When Frederic Larsan asked him for information on this point, he quietly replied that it was no business of his how he spent his time in Paris.

On which Fred swore aloud that he would find out, without anybody's help.
"All this seems to fit in with Fred's hypothesis, namely, that Monsieur Stangerson allowed the murderer to escape in order to avoid a scandal.
The hypothesis is further substantiated by the fact that Darzac was in The Yellow Room and was permitted to get away.

That hypothesis I believe to be a false one .-- Larsan is being misled by it, though that would not displease me, did it not affect an innocent person.

Now does that hypothesis really mislead Frederic Larsan?
That is the question--that is the question." "Perhaps he is right," I cried, interrupting Rouletabille.