Sirius had taken it upon himself to revisit every passage and secret room, corridor and cupboard that they new. Partially because a clue may be around there somewhere, partially to make sure they were all still the same, but mostly because he was bored and exploring the castle was a tried and true pastime.
He heard someone running by, and wondered if James had decided to come with after all. Though why James would be in the basement he didn't know. He was about to call out to him when an unfamiliar voice said "pardon me!" And an absolutely adorable stranger in Hufflepuff pants came bolting up the stairs.
"Woah! Slow down mate, no one is chasing you! Let me guess, you were getting ready for bed and then suddenly appeared here. Am I right?"