"Sherlock, don't panic but I think we've got a tail." John's voice was quiet, but surprisingly relaxed.
Instinctively, Sherlock grabbed John and pulled him into an alley, flattening them both against the wall just in time for a haughty-looking, elegant black cat to follow them.
Sherlock glared at the cat and rolled his eyes at John before dragging him back out of the alley.
By the time they made it home, the stubborn animal had followed them for over a mile. It flopped down on the front steps of 221 Baker Street, struck a rather indelicate pose, and yowled at John and Sherlock.
Something about the creature's sprawling posture and vaguely irritated expression reminded John of a certain flatmate, and he couldn't bear to leave the poor thing outside overnight.
Once in, Sherlock bonded with the cat immediately. Certainly not enough to remember to feed or care for it, but enough that they often formed a cohesive unit when it was time to harangue John.
A few days later, they received a call from a distressed older woman, asking if the famous detective could help find her prize-winning show cat. It was the easiest job they'd ever had.
John was glad to be rid of the thing. He had grown tired of being looked down upon by both man and beast.