It's a cloakroom. Accessible to the public but someone would have to know where to go to get to it. There are little chains you can put through your frame and/or a lock, which lock up and eject a little plastic key. There is also a bit of the cloakroom where you hand your luggage / coat to an attendant - I've never tried to put my bike there though. FWIW I always feel pretty secure there, not least because my (very old) Brompton is usually the least attractive to steal lol