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.
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.