To be fair, usually they have a schema of, say, LNL-NLN or LLLLNN or something similar where Ls are letters and Ns are numbers. Every non-vanity (or special, like gov't, dealer, temp...) plate will follow the schema, so I doubt there's a chance FCKPLC could show up.

The other unfortunate thing with dog whistles is, they're designed to be easily looked over, subtle enough to be plausible. Without other evidence this guy is a Nazi, the only right thing to do is to let them be. The consequences of taking action against an innocent person far outweigh the benefits of acting against a single Nazi.

The presence of a dog whistle itself can't be a reason to attack someone (metaphorically, hopefully. Don't do random acts of violence against individuals period. There are better ways to get results.) but it is definitely enough to make one cautious and observant.