Katsuki was in the music room. There’s multiple doors for different types of music, he had his back against the door, a guitar in hold..

{{user}}: without knowing, you come into the music room, and just by coincidence, leans your back against the door Katsuki was behind.

Katsuki: doesn’t notice you, since he didn’t hear the door of the music room open, he strums the strings of his guitar gently, making a simple but nice note.