While in Europe I say bikes with a lock that looked like an "o". it was mounted on the back side of rear brakes on the Seat stays. The lock was not something you could remove it stay on the bike 100% of the time. You rode it with the lock arms open. It would lock the rear wheel to the frame and make the un-rideable. You could use it without having to have the bike in a rack. I have also seen it called a “nurse’s lock”

Cheers