Possibly slightly controversial, but I don’t actually see a major problem with this.
The hirer is resposible for the security bike when it is not in a dock & they have therefore taken measures to secure the bike whilst not in use. They may have just popped into a shop to buy something! How long was it there? Did you wait to see if they returned?
The hirer is also liable for hire charges when the bike is out of a dock. So they could be riding it, sitting on it, taking it out for dinner or even riding it from here to Timbuktu – it doesn’t matter. They’ve simply got to be prepared for a hefty usage charge if it’s for any significant length of time.