"Someone on Facebook pointed out that the Ofo bikes use a number code which does not change, and some users claimed to have taken the Ofo home after their first ride and used it for free thereafter."

There are a lot of these 'stolen' Ofo bikes in Beijing. They usually have an additional regular lock to prevent other Ofo users from unlocking them using the code (which the app will provide, so long as the bike isn't reported as currently being used).