
When Potter was done killing Voldemort, he didn't hang around long.
One hasty graduation ceremony, seven funerals, thirteen hearings, and thirty-six days after Tom Riddle fell, Harry Potter flew into Denver, Colorado, rented a pick-up truck, drove to the foothills, climbed into the Rocky Mountains, and disappeared. He was seventeen.
Severus Snape did not miss him.
Reinstated at Hogwarts, reluctantly cleared by a pressured Wizengamot, and still held as a traitor by much of Wizarding society, Snape perversely wished that Potter had not bothered to stick around long enough to testify on Snape's behalf at the Wizengamot. Without the testimony of the hero of Wizarding Brittain, Snape probably would have been chucked in Azkaban, physical evidence of his true (virtuous) allegiances notwithstanding. Then, at least, he would have known where he stood.
.....
Hogwarts was mostly rebuilt within a year--no thanks to the ineffective bureaucratic fumblings from the ministry. Most of the real work was performed by the school itself, which healed the wounds in its walls at about the same pace that Snape's torn leg healed: slowly, but not imperceptibly. By the following spring, ten months after Potter finally did what he was supposed to and then, mercifully, buggered off, the roof was almost whole and Snape was nearly walking on his own. He used a cane and had a pronounced limp, but he was once again free to roam the halls at night.
Searching for nocturnally wandering miscreants had lost some of its (admittedly petty) pleasure, however. For one thing, most of the potential miscreants had been significantly traumatized by the previous year's bloodbath. The students of Hogwarts now generally cowered in their beds at night, too wary to go looking for trouble. It was also possible that Snape's heart was not quite in it. After a few dutiful nights of lurking in blackened corners for mischief-makers that never came, Snape spent the entire night in his bed for the first time in many, many years. It made him cross.
His sleep was plagued with the expected nightmares and ghosts, but there was something worse. Some vague, horrible, nagging unease that he couldn't quite put his finger on. Snape lay in bed at night and recited potions recipes and classic defense strategies, alphabetically by the dark creature they warded off or destroyed. He usually made it at least as far as Sirens before dropping off.
....
The ceiling of the great hall was mostly showing charmed sky, with diminishing gaps where the real sky showed through.