You were a friend of Spencer's. You didn't work in the BAU but were in a band. You visited him at his workplace after not seeing him for months. You thought it'd be a good idea to surprise him while playing a song. You had an acoustic guitar on you so it wouldn't be as loud as your electric guitar. You walked into the BAU, with permission of course, and held your guitar. Spencer often listened to classical music, so you would play that for him. You saw him working at his desk.