I've come across various cases of this too. For example, I found a passage concerning the baptism of the Israelites in the Red Sea where Turretin almost certainly copied a portion from Gataker without attribution. I'm not sure, though, that back then this kind of thing would have been considered plagiarism in the same sense it is today. In the case I mentioned, the Latin is somewhat different, but the subject-matter, sentiments, and even sequencing are the same.
I think we can say with a high level of confidence that the standards for attribution and copying were vastly different from what we have today.
In fact, specific attribution, well into the 1800s, was actually DISCOURAGED when criticizing opponents who were considered to be "within the camp," with the idea being that "friendlies" shouldn't be singled out too severely. I remember reading a polemical book from the 1700s by a minister in one of the separatist Presbyterian denominations who was ministering to his flock in America -- I am no longer 100 percent certain but I think he was an Associate Presbyterian, then known as the "Seceders," one of the groups that later became the Associate Reformed Presbyterians -- and criticizing the other bodies (what today would be the RPCNA or Covenanters, the Old Side Presbyterians, and the New Side Presbyterians). He made reference to their distinctive doctrines in detail but never once mentioned the denominational name or the specific writer he was criticizing. It's likely that when he was writing, his targets and his readers would have known who he was talking about, but without footnotes or far more knowledge of back-country Pennsylvania Presbyterianism of the 1700s, I was completely unable to identify who he was criticizing.
Even in the 1900s, Machen made reference in some of his writings to a Roman Catholic speaker whose opposition to liberalism he appreciated. For three decades, I had thought Machen was referring to Fr. (later Archbishop) Fulton Sheen, a Roman Catholic apologist who was well known for attacks on liberalism and for encouraging Catholics to read conservative Protestants and their Scriptural exegesis. It's possible. But if it was Fr. Sheen, it probably would have had to be when Machen was studying or vacationing in Europe since Sheen's American work was mostly after Machen's death, and during Machen's lifetime, Sheen was mostly studying and preaching in Europe, and -- much like Machen -- was horrified by the growing damage done by liberalism to his church and wanted to keep it out of the United States. This is the sort of thing that today we can easily check using the internet, but for someone like me who lives in rural America and hasn't had access to a major theological library since leaving Calvin Seminary, it would have been very difficult to know the timeline of when Machen and Sheen could have interacted without the sort of biographical information that is today available by a few clicks of keys, but back in the 1980s and early 1990s, would have required a great deal of research in a major library.
Now imagine the problems of attributing things when every new edition of a printed book had a new set of page numbers, and people were still using hand-copied manuscripts, and only the very largest universities had anything close to a modern theological library, and even then, there were huge gaps and scholars might have to travel hundreds of miles to read a book they didn't have in their university library.
Attribution not only wasn't done; it simply wasn't possible.
And that's not even taking into account the problems of printers errors, damage to hand-copied manuscripts, and scribal errors.
The modern standards of plagiarism simply were not possible prior to the widespread use of modern printing techniques, (relatively) low-cost books, and large library collections.
