Sirius took the bike to Hogwarts. His new broom had failed weeks ago, but older charms seemed to work better than newer ones, as if they had soaked in over the years and couldn't be as easily pulled out. He flew low just in case. At Hogwarts, he left the bike with Hagrid, and strolled up to the school.
"Sir?" a bold Hufflepuff prefect asked, and he winked at her.
"Just looking for Professor Blank. He's usually home by now. Do you know where he might be?"
"Er, no sir, but --"
"Would you show me to his office, then?" Sirius didn't actually need help getting to the office, but escorting him would no doubt make this poor girl feel that she had used due vigilance in the protection of her fellow students.