Germany's Polizei are not US police and don't have the same restrictions placed upon them. For instance:
A US servicemen who was described about 6ft, male with short close cropped hair raped a women in Germany. The polizei knew his name from the victim. They entered a club in Stuttgart and proceeded to check every single male's ID that fit the above description. Which is pretty much all of them. One guy, who made the mistake of thinking the police in Germany had to follow the same rules refused to show him his ID. They simply pulled out their baton (a spring loaded club) and beat him to the ground. The polizei then reached into his pocket, pulled out his ID and inspected it. He was not the person they were looking for and they left him laying in a heap on the ground and moved to the next person.
You do not tell the police in Germany that you will not comply with showing proof of who you are. You don't have the right to refuse items like a blood test if they ask for one from you.