In the book Harry Potter and the Order of Phoenix , we see Dolores Umbridge quoting the following.
“Very well, Potter, I will take your word for it this time, but be warned: The might of the Ministry stands behind me. All channels of communication in and out of this school are being monitored. A Floo Network Regulator is keeping watch over every fire in Hogwarts — except my own, of course.
And later we see Harry using this knowledge, that Umbridge's fire was the only one not being used by the ministry, to communicate with Sirius and later to check if Sirius is in home.
“Umbridge’s office,” said Harry quietly.
He had been thinking about it for a fortnight and could think of no alternative; Umbridge herself had told him that the only fire that was not being watched was her own.
Looking at the way Umbridge mentions about her fire not being watched, Doesn't it seem obvious that this hook was created retrospectively, so as to pave a way to the climax of the story?