You can be captured by video, laser camera, non flashing camera and the classic speedtrap. Tolerances vary, its about 5-10% and violations of 5-10kmph inside and 15-20 outside town attract only minimal fines and no specific police procedure. Often regional enforcement stops pursuing such violations when the rental company informs them that the driver lives outside of Germany. The postage and effort to get a 10-15€ fine for example are deemed disproportional. All rental companies nowadays charge 12-30€ to process such enquiries, so it hurts either way.