Welcome to the site, as for the rozzers, from what I can gather it seems as though they take offence to german number plates when they stop you for something else and give you the once over (in a non physical way obviously!
), you'll probably be unlucky if they pull you over just for the plates, but hey, you never know. Have a legal plate to hand and if you get pulled just say that you've been to a show at the weekend and forgot to take them off but you will do as soon as you get home.