those dogs rarely sniff anything, they are rather following hints from their trainers.  The true sniffer dogs are used behind the scenes to sniff bags. not people. The policemen didn't like you for some reason and wanted to single you out. They probably knew they wouldn't find anything, but wanted to make go through all the hassle.

I'm not sure how it works in Germany, but If you have the time, you should consult a lawyer and sue the crap out of them for harassment.