I heard many cases of LLMs outputting their training data verbatim; they sure don't check that such data can be legally used by you before doing it.

As others said, for closed source software it's a lot harder to detect plagiarism.

And it's in general very hard to detect plagiarized code; there's some product that does it, but it of course has only a partial inventory of the existing open-source code.

I think that some big company does use those products, though.