Disclaimer: I do not own any Harry Potter characters or books. Otherwise this would be a book and Sirius would not be dead.
Not Dead Yet: Gone, Gone, Gone
Once every one realized that Voldemort was actually dead, they cheered. Despite all the dead, it was a relief for it to be finally over. Death Eaters suddenly stopped, confused about what to do. Some still fought, though they were easily repressed due to the Light Side's renewed vigor
But between the last of the battle, mourning and burying the dead, and taking care of the wounded, no one noticed their savior leave. Just like that. Gone. Not to be seen again by anyone from their world for another ten years. Search parties had been sent out when they realized, but he was gone. Some thought he was dead, others that he had been kidnapped by Death Eaters to avenge their master.
Not one person guessed right. What really happened was he left, simple as that. He was done with being the Boy-Who-Wouldn't-Die. So he left the Wizarding World, only venturing there to deal with his property and money. The goblins took pride in that the savior trusted them to keep his identity a secret, so he was safe in that respect. Besides that, he lived like a Muggle, albeit a rich Muggle without a job.
The neighbors liked him well enough. Though why he was always so sad at such a young age was beyond them. He had moved into the small house by the dead-end. The house had a kitchen, a bathroom, a bedroom, and a small sitting room. He lived there alone. There was no Mrs. Potter.
Despite all this, he was a nice, quiet young man. He donated a generous amount of money to the schools and libraries. Girl Scout Cookie season would come about and his was the first house they went to. Children found he was a good listener and could bring out the cookies while they talked and did their homework. Every Christmas all the kids would get a small present from Mr. Potter.
Other than that he was a little on the short side, but was lean and muscular. Wild raven black hair hid his now dull green eyes. Almost everyone called him 'Uncle Potter.' None of them knew he was also depressed to the point of being suicidal. Not that he needed to kill himself, he was dying anyway.
Apparently, you can't survive the killing curse two times and live normally. He would tire faster, and casting the simplest spells made him weak beyond belief. He wasn't all that upset about dying, he welcomed it even. All he wanted to do was say goodbye to the place he called home before departing from this world.
It came as a surprise to everyone when they found he left in the night. After all, he had lived there for ten years. His lawyer came knocking on all of their doors, leaving outrageous amount of money as well as gifts that Uncle Potter had left to them. Nine year old Nick Thistle got his owl, which came with a note saying he would need her in a couple of years. Harry Potter never came back.