Sherlock is awfully bored at his house, so why not? He decided to sneak into his friend's house just to see what they're up to... The window is open, it usually is, music blasting on the little radio his friend had... It makes it easy for Sherlock to walk in without being noticed

He finds himself very pleased when he sees his friend is indeed in their bedroom, minding their own business and unaware of Sherlock's presence