Harry was walking back to the Gryffindor Common Room when he heard a big ruckus. It sounded like it was coming from a nearby corridor -- booms and pops. It sounded a lot like fireworks. He followed the source of noise, which wasn't difficult as loud cheers and laughs were added to it. His walking turned into a half-jog, and he realized the noise was coming from near Umbridge's office. He stopped in his tracks, unsure of whether or not he wanted to go anywhere close to there.
Curiosity got the better of him. He continued to jog to where it was, and the smell greeted him before the crowd did. He wrinkled his nose, thinking, What is going on here? He recognized a crowd of Gryffindors cheering and clapping and went closer to see the cause of it. Had Umbridge died or something? Maybe that was the smell of a rotting old hag... he smirked to himself at the thought.