If you work for the government, your employer has the option to register your secondary activities and make them public. This is stated in the Central and Local Government Personnel Act. Organisations outside the government can also implement such a policy. They do need the consent of the works council for this.
However, your employer may not oblige you to report all your paid and unpaid secondary activities. This must be limited to the necessary positions and relevant secondary activities. It is also not necessary to disclose all information provided, such as the income from secondary activities.