And Snape had no doubt that this same group was behind the theft of his private stores. What he still didn't know was what connection they had to Voldemort, if any.
"It can't be that idiot Quirrell. So who, then? And where are they hiding? Some other professor, perhaps?"
Snape turned the events of the past weeks over in his mind, weary down to his bones.
A chill crept over him out of nowhere.
His eyes snapped open, one hand already halfway to his wand.
The fire in the hearth surged and flared, shuddering, then twisted into an eerie shade of green.
A handful of specters slid out through the wall. At the sight of Snape they seemed to freeze for a heartbeat, and then a wave of vicious cold burst outward from them all at once.
A basic spell that came naturally to undead spirits, not much of a threat on its own. Much like the ambient cold a Dementor gave off without even trying.
A layer of white frost spread rapidly across the room, followed by the sharp crackle of things freezing solid.
"Expecto Patronum!"
The instant Snape saw the specters, he assumed Dementors, and the moment that cold hit him, he was sure of it. He cast the Patronus Charm without a second's hesitation.
A doe made of soft silver light leapt from the tip of his wand, and its glow flared bright.
The specters weren't actually afraid of the Patronus Charm, but that warm, guardian-bright light was still enough to unsettle them. They let out a series of shrill, grating shrieks and retreated straight back into the wall.
The cold in the room slowly bled away. Snape's face had gone the color of old iron.
"What in the blazes? What are Dementors doing at Hogwarts? And since when can they walk through walls? What is going on here?"
He didn't waste time puzzling it out further, wand in hand, and took off after them.
The "Dementors" drifted through the air ahead of him, heading straight for the fourth floor.
"The fourth floor?"
Snape's expression darkened. His mind jumped straight to the Stone, and he quickened his pace, wary now.
At that same moment, Harry and Ron had just come down from the eighth floor and were creeping carefully toward the room at the very end of the corridor.
The two clouds of black mist churned like boiling water, and combined with the snarling ghost masks, the effect was genuinely frightening.
"If Filch caught us patrolling looking like this, he'd drop dead of fright."
Watching Harry, Ron could easily picture what he himself looked like too, and he found himself hoping George and the others would keep well clear of Filch, and that Filch wouldn't wander up to the fourth floor tonight of all nights.
They reached the door. The lock above the handle looked intact. Harry thought it over for a moment, then drew his wand.
"Alohomora."
Nothing happened.
The truth was, when Snape had come to check on the chamber earlier that day, the sight of the lock had reminded him of his stolen collection, and he'd gone ahead and layered on a string of anti-theft and counter-unlocking charms just to be safe.
An ordinary unlocking spell wasn't going to cut it now.
"It won't open..."
Harry sounded thoroughly deflated.
"Am I really that much worse than Hermione?"
"We're only here to check the thing's still in one piece, anyway..."
Ron said it with a shrug, then pressed his ear to the door.
"Listen... you can hear that dog snoring..."
Harry leaned in beside him, straining to catch the sounds from inside.
"I believe you two owe me an explanation for why you're out here."
Snape's flat, emotionless voice came from directly behind them.
Harry was sure he'd imagined it, and turned to Ron.
"I could've sworn I just heard that git Snape..."
"I think I heard it too... bloody hell..."
Ron had barely gotten the words out before he froze solid, and both clouds of black mist shuddered violently as the both of them whipped around.
Snape stood there with his usual chilly composure, wand in hand, staring straight at Harry and Ron.
When he'd chased the specters up to the fourth floor, Altair had already called them back, so Snape arrived to find no trace of them at all. He noticed the icy presence had simply vanished and assumed they'd moved on somewhere else.
He'd been about to turn back when the unmistakable sound of an Alohomora charm drifted down from the end of the corridor.
And the voice that cast it, unmistakably, was Harry's.
So Snape had crept forward without a sound to see what was going on, and found two roiling clouds of black mist waiting for him. He'd very nearly cursed first and asked questions later. If Ron hadn't blurted out Harry's name at that exact moment, a hex would already have been on its way.
"Professor... Snape."
A wave of pure dread washed over the pair of them.
This was the second time they'd snuck out at night.
And the second time they'd been caught.
The first time it had been McGonagall. This time, even better, it was Snape.
"Take that rubbish off your faces!"
Snape had already recognized who he was dealing with, which meant he'd also worked out at a glance exactly where the black mist was coming from. He snapped the order out sharply.
Harry and Ron both flinched, and pulled the masks off, revealing themselves underneath.
"Mr. Potter, Mr. Weasley, you had better explain what the two of you are doing wandering around here in the middle of the night."
Snape's gaze settled on Harry, cold as ever.
He wasn't sure what to make of it.
Did Harry have anything to do with those Dementors just now?
And where had they gone?
Had the whole thing been staged just to draw him away and catch Harry and Ron out of bed?
None of it added up.