When the first war ended, everyone knew who the death eaters were. The ones who had not been Imperiused, and did not rat out other Death Eaters were given trials and sent to Azkaban. So how did no one know Snape was a DE?
At first, Harry had asked Hermione about the Knut ten times a day. Once he realized nothing had happened, and nothing probably would happen, he practically forgot about it completely. It may be years before Voldemort could find a way into Hogwarts. Dumbledore was right, Harry shouldn't worry about it at all. Harry aced his History of Magic and Defense against the Dark Arts finals. He was pretty sure he did well in most of his other finals, but he bombed the Potions final. He wasn't too worried, though: Since most Gryffindors tended to fail Potions class, Snape was forced to grade on a curve, so Harry thought he had a good chance of passing. After exams, the students had a full week to relax before getting their final grades and returning home.
On Wednesday night, just before Harry and Ron went to bed, Hermione ran up to beckoned them to a sofa in the common room far away from prying ears.
"My alarm went off. About thirty minutes ago, Professor Quirrell opened the door."
Harry could hardly believe that the timid Professor Quirrell was in league with Voldemort, and said so.
"Maybe not", said Hermione. "But five minutes later, Professor Snape went through as well. ThenI went to go talk to Dumbledore about it, and asked Professor McGonagall how I could find him to ask about grades or something, and she said Professor Dumbledore just happened to be called to London on urgent business. Something is going on, Harry!"
Harry's blood ran cold. Everyone had that one teacher that you knew was evil, but you didn't think they were that evil. Sure, Snape had treated every Gryffindor with disdain, and did not give even Hermione an A on any tests. But could he really be in league with Voldemort?
"Look, I know it doesn't make much sense. But is it just a coincidence that Dumbledore was called away to London right before Quirrell and Snape entered the forbidden corridor? It may seem unlikely that Quirrell would be evil, but if anyone here would support Voldemort, it would be Professor Snape. He even used to support Voldemort before Dumbledore supposedly turned him and made him a spy."
"Well, why didn't you tell Professor McGonagall?"
"Why didn't I tell her that two professors are secretly servants of Voldemort, who is supposed to be dead, and that I know this by performing a spell which I shouldn't be performing yet, and I was able to perform it by using an invisibility cloak which we shouldn't have and being in the corridors when we should be asleep? Is that what you are asking me?"
"So should Ron and I pack our bags for Australia now?"
"No, silly. It is possible, after all, that they went in there under Headmaster Dumbledore's orders. We should go find out!"
Harry and Ron looked at her with amazement.
"I think the lack of tests to study for has made her go crazy", Ron told Harry.
"I'm serious", said Hermione. "We won't do anything terrible. We'll just open the door, and see what is going on. If all looks good, we'll pretend nothing happened. If not, we'll leave and tell Professor McGonagall."
Harry and Ron could see the logic to that. Harry got his cloak, and the three walked as quickly as they could without looking suspicious to the third floor. They ducked into a narrow corridor, put on the cloak, and scurried to the locked door.
"Alohamora!"
The door sprang open. It was pitch black inside the door, as if a black veil was covering the opening. Hermione inched forward, and the other two followed. As soon as they crossed the threshold, the veil was lifted and they realized why the corridor was forbidden. They turned to run, but the door slammed shut behind them.
The corridor was not very long by Hogwarts standards, probably 100 metres long, and perhaps a third as than the door they came through, the only other door was at the other end of the corridor. None of this was noticed by the three, though, as the enormous, 15 metre tall three-headed vicious, snarling dog took up all their attention. It was held by a long chain attached to the back wall, which is possibly the only reason they were not dead yet. It was clearly guarding the back door. Harry started yelling spells to try and open the door, while Ron just kicked it. Hermione, however, started singing. Hermione may be excellent at magic, but apparently she was terrible at singing. Ron and Harry stopped to stare at her, when she started to 'sing', in the same awful tone she was in before,
"It's a hellhound, they are charmed by music, look at it please..."
Harry looked at the dog. It already seemed to be a bit droopy. Not long after, it fell fast asleep. Harry tried the door again, but it was still locked.
"It looks like we have to go forward, then", he said. Hermione, still singing, nodded, and they all tiptoed past the sleeping creature.