If I may note a slight inaccuracy in the question: when someone uses plagiarism check software, what it finds (or doesn't find) is not exactly plagiarism. It looks for long runs of similar content between the work being checked and other works in its database. If a suspiciously high degree of similarity is found, that can be used as evidence to support an accusation of plagiarism - or, more relevant for this site, an accusation of copyright infringement (which is something different from plagiarism, although they often occur together).

To give an example of where plagerism checking can flag perfectly valid papers, I once wrote a term paper that had a lot more pings than I had anticipated. Turns out, all of them were contained in my works cited page... they were hitting on other paper's identical citations.

@MonkeyZeus If no one in your neighbourhood is named MonkeyZeus, doesn't that mean you are the only MonkeyZeus in the neighbourhood? No? If you are not aware of another MonekyZeus and it is not in the database of record, is it not acceptable to believe you are the only one in the neighbourhood?

@Skies No, per my comment it would be unwise to claim that I'm the only MonkeyZeus in existence when my search scope was just neighborhood.

@MonkeyZeus Right. if there is a defined search scope.

@Skies What plagiarism detection software are you using which guarantees to have a database of every single work under copyright, including works which are not publicly or widely distributed, e.g. a private e-mail?

@user71659 Did I say is me? Did I say the plagiarism detection software guarantees to have a database of every single work under copyright, including works which are not publicly or widely distributed, e.g. a private e-mail?

@Skies You said "database of record". So the "database of record" would necessarily contain every work under copyright anywhere in the world, including non-public works. That's the "defined search scope".

@user71659 "database of record" means "every work under copyright anywhere in the world, including non-public works"? This is taken out of context. I'm not going to continue on this conversation further. Thank you.

@Skies I think you have fundamental a misunderstanding that copyright requires some sort of registration or deposit. After the Berne Convention in 1989, it no longer does. A private e-mail or photograph is copyrighted. Again, how is the plagiarism software going to detect you illegally copied a photo I texted you? It can't. Which is what we're telling you. (Trademarks are similar, but there is a database of valid patents)

I once asked someone to start over on a school report. I explained that copying a Wikipedia article and then rewording every sentence is still plagiarism even though the AI probably would not detect it. Obviously, the human (me) did detect it!

AI plagiarism is a whole other can of worms that hasn't been legislated all that much as far as I know. Being only a user of AI may provide some legal protection, but that wouldn't be absolute (and you're still probably going to be on the receiving end of all the lawsuits involving what you published, even if you have a good enough legal defense).

"A plagiarism checker" doesn't mean much. I can write a plagiarism checker which only checks plagiarism against exact matches on the works of Shakespeare (and returns false for everything else). So you'd really need a checker that's officially endorsed or recognised by the government or the legal system if you want to legally rely on it, and you'd also need to pay attention to any caveats of said endorsement. But I'm not aware of any such endorsements, which isn't surprising, given that similarity and plagiarism are related, but distinct (and plagiarism checkers check for similarity).

The AI company that provides the software has marketed to provide plagiarism free copies. No, their marketing will very carefully say something like "the most accurate* plagiarism detection software available", and the EULA will explicitly disclaim any and or responsibility for errors on the part of said software. Ultimately, the person responsible for ensuring plagiarism is not committed is you.

May we concentrate on the Question? How could one be free from legal responsibility? How could plagiarism-check software be better-qualified than for instance your secretary… even a plain old, let alone a specialist legal secretary? How could 'My secretary failed to tell me the Governor called, asking about some detail that would have given my client a let-out, so the poor guy was put to death' be a defence? It puts the blame on the secretary but how could that excuse the department, the office or the boss?

Plagiarism and copyright violations are two different things.

@NotThatGuy Right, to prove the work is or is not AI authored(partially or fully) is not a walk in the park.

@Skies The question is more what AI authorship even means, whether we decide AI is creating "new" works by looking at other works (like a human would) or whether AI is simply creating derivations from other works (like Photoshop).