If it has "no clear copyright owner" because it comes from an entire community and has a ton of copyright holders for little bits of the code here and there, then it means that every copyright holder can sue for breach of copyright.
If on the other hand it has "no clear copyright owner" because it was written by one person under a pseudonym and that holder is almost surely never going to come forward to prove that they are the person intended... then it's de facto public domain even if that's not its status de jure. If you really want to push it in court, you can say that you met the person in-person at a conference and that they verbally told you to do anything you want with it: unless they stand up to say "that's a total lie!" there's no way of proving, because just because some code C has been licensed to a million people under the GPL, it doesn't mean it hasn't been licensed to one other person under the WTFPL.
As for how this could happen... It could be that everything suggests that they never want to come back, like the apparent disappearance from Earth of Satoshi Nakamoto; it could also be something with more stake, for example if Ross Ulbricht had gotten out of the Silk Road business and GPLed his code as the Dread Pirate Roberts before he was caught; in that hypothetical universe, nobody would want to prove that they were the DPR until some statute of limitations for running the black market had passed.
¹I believe there are some exceptions, like having contributed only an insignificant fraction of the whole work.
Let's say you contributed 500 lines of code to some project. You sue somebody for violating GPL and not publishing their changes. The judge has to give them the choice : either pay damages or actually comply (another western legal principle : money buys your way out of anything, as long as it isn't criminal).
What damages to assign ? Well, seems reasonable to say "how much for 500 lines of code ? A week's wages ? Okay, 2500 dollar damages awarded". The other party might become scared because other developers could conceivably sue, but ...
1) are only available either during a trial or as a last resort (if you can make a good case the defendant won't pay even if the judge compels them)
2) you'd still have to prove more damage will be done if injunction isn't there. Those have to be real damages, with real monetary amounts on them.
This is western law, it's practical. Anything you do that either doesn't bother anyone, or you prevent damage from occuring is legal, no matter how much it is against (civil) law.
The fact that something violates copyright, by itself, is not sufficient to get anything done in court at all.