Harry receives a warning from Sirius about meeting places

After a noisy evening in the Gryffindor common room, Harry, Ron, and Hermione meet with Sirius via the fireplace. Sirius warns Harry that they need to be more careful about where they hold their Defense meetings. Sirius reveals that Mundungus Fletcher had been keeping an eye on them during their Hog's Head meeting, disguised as a veiled witch. He advises caution but supports the idea of the secret group.