Sherlock was wandering the halls, having grown somewhat bored of the library. Yes, the books were interesting but there was very little in them that would be of practical use to his work when he returned. So he'd deleted almost everything he'd read.
Of course, he had something else to occupy his time now - John - but sex with John couldn't entertain him every minute of the day. So he was wandering the corridors, looking for a distraction.
He rounded the corner and collided with a man. Tall, dark haired. American, judging by the clothes and teeth.