The security of medical radioactive sources, both open and sealed, is an important consideration for reducing the risk of an intentional or inadvertent additional radiation dose to the public, according to the principle of keeping any additional radiation dose as low as reasonably achievable. The detection and following radiological investigation of the misuse of iodine-125 (I), a medically used radionuclide, in Germany is described in detail with the aim of sharing experience and raising awareness. The misuse of I shows that the security of I is not guaranteed completely at the present time.