Sirius stayed in the Gryffindor common room for another month or so after the Aragog incident. But then the teachers started getting mad at him.
"He keeps telling the students to do irresponsible things!" said McGonnagal.
"He tracks mud in the halls!" said the janitor-guy (forgot his name).
"He's scaring me a little", said Neville.
So, it was final. Sirius had to go back to Grimmauld Place. He said goodbye to all of his friends at the school, packed his bags and left-
-To find that some of the order of the phoenix had taken up residency in his house.
"Tonks! Lupin! What are you lot doing here? This is my home, I don't recall saying you could live here!"
They looked sheepish. "Well, ol' pal of mine, if you recall, it said in your will-"
"My will! I'm not dead yet!"
"But, you were declared legally dead last year." Tonks said. "Everybody's gone off with your stuff. Have fun tracking it down."
"Oh, and we're getting married. Did you hear?"
"No! That's great news! About your marriage, I mean, not my stuff."
"You can live here if you'd like", Tonks offered. "Although, you might not want to use your old bed anymore."
"Why? WHAT HAVE YOU TWO DONE IN MY BED!"
They gave each other a sly look. Sirius decided he didn't want to know.