Good point, many of the service center chargers are not "destination chargers" and not public.You may have to ask to use those destination chargers. I wouldn't just roll up and plug in. They are probably meant to be used for vehicles out for delivery or in for service.
For example - there are Tesla Wall Connector chargers at the north Austin service center, but that doesn't make them destination chargers and they aren't listed as destination chargers on the Tesla site.