During a particularly long lull in between cases, Sherlock was looking for ways to relieve his boredom. He was miffed to find out that John has locked his gun away. He decided to retaliate by waiting until John was in the shower, then sneaking into the bathroom with hos violin and screeching out the shower theme to Psycho. John was so startled that he slipped. The police station was soon flooded with calls from the neighbors about a crazy man in a towel yelling and chasing someone down Baker Street.