Excuse me, but that is not entirely true -- you're mixing up two different things. Yes, there are vans that can detect certain kinds of EMR but this has nothing to do with the obligatory fee you have to pay for watching (public broadcasting) TV. It's more about locating pirate radio stations or faulty/unlicensed electronic equipment that is interfering with TV reception or other things. That the GEZ, the agency which collects the TV/radio fee, has TV-detection equipment, too, is an urban myth.
With regard to the grandparent: I guess the government could easily detect wifi routers; but given the sheer number that doesn't sound practical. I guess in the end it will be similar to the current situation: you can have an open access point but you are made legally responsible for any abuse that is done via your internet connection. Which is the reason why the number of open APs is drastically smaller than in the US.