When I had a mk3 gs300, i did the same as yourself, and it came up with cd2, so I thought someone had added another cd changer, I checked the boot etc and nothing, then opened the arm rest console, and deep down I saw an aux cable someone had fitted, maybe previous owner, plugged it into my phone and I could play music from it. So every time I wanted to use my phone to play music, I had to press on cd2. Maybe someone has fitted aux cable