Remember, dummy cameras are subject to the same legal requirements as functional surveillance cameras.
Is that supposed to be a joke? – a dummy neither records footage from beyond the fence nor stores it – or did you mean that a complete dummy also includes the sign "video-monitored area"?
It is probably about the generated pressure of surveillance. This can also be created by decoys, since the supposed "observed" person knows full well that it is not a real camera and still feels restricted in their freedom and feels watched...
Paraphrased from a decision by the Federal Court of Justice: "that neither adjacent areas nor public spaces are targeted by the 'wrong angle.' Exceptions exist only for apartment buildings with multiple units." The wrong angle is the supposed surveillance area of the dummy. I dealt with the issue when graffiti was sprayed on our uninhabited demolition house. Hey, we live in Germany...