Why didn't Sirius remind Harry about the mirror when he used Umbridge's fireplace?

Question

I understand from this answer why not-so-bright Harry didn't understand the true usefulness or significance of the two-way mirror until it was too late. This was basically because Sirius described it to him as an emergency method of contacting him if and only if Snape was giving him a hard time.

"What is it?" Harry asked.
"A way of letting me know if Snape's giving you a hard time. No, don't open it in here!" said Sirius, with a wary look at Mrs Weasley, who was trying to persuade the twins to wear hand-knitted mittens. "I doubt Molly would approve - but I want you to use it if you need me, all right?"
"OK," said Harry, stowing the package away in the inside pocket of his jacket, but he knew he would never use whatever it was. It would not be he, Harry, who lured Sirius from his place of safety, no matter how foully Snape treated him in their forthcoming Occlumency classes.
_{(Order of the Pheonix, Chapter 24, Occlumency)}

Here we can see that Harry is worried about Sirius' increasing recklessness. But he also doesn't understand that the mirror will work as a general communication device. So when he wants to contact Sirius for other reasons - to talk about the Pensieve and to enquire as to Sirius' whereabouts - later in the book he thinks that the only way to talk to Sirius is to use Umbridge's fireplace.

Harry thinks this but Sirius knows better. Why didn't Sirius ask Harry why he wasn't using the mirror that Sirius had given him for the exact purpose of getting in touch? Surely he should have been bewildered as to why Harry was using the much more risky method of Floo powder? Is there a good reason why Sirius didn't effectively say, "Let's continue this conversation using the mirrors?". Obviously hindsight is a wonderful thing but it would have saved a lot of trouble later on.

Answer 1

We can't really know why Sirius did not ask Harry why he was not using the mirror, but I can think of a few ideas:

1. First, Sirius did not know that Harry had broken into Umbridge's office to contact him. Harry did not mention whose fireplace he was using.

   One might imagine that Sirius would have assumed that Umbridge was watching the fireplaces from his earlier experience:

   “Oh . . .” said Sirius, frowning. “Well, I’ll have a think and get back to —” He broke off. His face was suddenly tense, alarmed. He turned sideways, apparently looking into the solid brick wall of the fireplace.

   —Harry Potter and the Order of the Phoenix

   However, as Hermione said in response to that incident:

   "Umbridge has been reading your mail, Harry. There’s no other explanation.”

   —Harry Potter and the Order of the Phoenix

   Umbridge most likely had seen the time Sirius was meeting Harry in one of his letters. Later, of course, she set someone to watch all the fireplaces, but it is debatable whether Sirius knew this:

   "A Floo Network Regulator is keeping watch over every fire in Hogwarts — except my own, of course."

   —Harry Potter and the Order of the Phoenix

   So there would not have been any reason for Sirius to assume that Harry was not using, say, the Gryffindor common room fireplace, and that (not having informed Umbridge) he was perfectly safe in doing so. Thus asking about the mirror would not have been a particularly high priority (particularly given how involved Harry was in the conversation).

2. Sirius may have assumed (from the position of someone very familiar with the wizarding world) that Harry would already have opened and recognized the mirror, and was thus choosing to use the fireplace for considered reasons. Again, given that Sirius had little reason to think that their communications were unsafe, he would not have particularly cared which form of communication Harry used.

3. Sirius may just not have thought of it. It was a short conversation (under twenty minutes), and they had a lot to talk about.

Answer 2

Sirius had to know the fireplaces were being watched as he was part of the order and certainly Dumbledore and McGonagal knew, and we know the order had means of communicating with each other, as Snape did when Harry went into the forest. Just a plot hole I think. JK needed some way for Harry to get caught by Umbridge communicating...