Suppose we have some text in a scientific journal article A. It is a verbatim copy of some text published in a related article B which is referenced by A, and this phrase is used to explain the same topic in both articles. However, the phrase itself is short, not a complete sentence and not very relevant (it could have been written in many different ways) but it seems the author of A has taken it from the original (B). Is this text plagiarism? I would rephrase it because it looks like a copy (it's unlikely it happened by chance given that the paragraphs in the two publications are related and the reference to B appears in a nearby paragraph) and that's not elegant according to my style and taste, but is there a general rule?
There is not a fixed set of rules for what is considered plagiarism (e.g., "5 or more consecutive, not quoted words"), although several guidelines are available. Unless entire paragraphs are copy-pasted, each case has to be evaluated on a single basis.
As for a partial sentence, it is likely not considered to be a bad case of plagiarism.
That said, it depends on what the actual sentence is about. Are those 500+ papers plagiarizing each others because of that partial sentence? Certainly not.
If the partial sentence was about a particular discovery reported in a particular paper, and that partial sentence was copied to a new paper as if the sentence belonged to the new paper, without a direct quotation.. that would likely be considered as a candidate for plagiarism. Quite for sure, it would be considered as plagiarism if a reference was missing, as well.
You might have some fun taking this Turnitin quiz on plagiarism.
"Suppose we have some text" (5 consecutive words), your first sentence, already yields 18 hits in Google. As you mention it is "not very relevant", and you know the origin, I suggest you to rephrase it.
There are no general rules, one reason why such matters are settled in courts. One clear rules though: "Give Credit Where Credit is Due". And many guidelines, like in Avoiding plagiarism, self-plagiarism, and other questionable writing practices: A guide to ethical writing, Miguel Roig.
Two additional food for thought:
- A common quote: If You Steal From One Author, It's Plagiarism; If You Steal From Many, It's Research,
- J. L. Borges (my favorite author) wrote a novel on "Pierre Menard, Author of the Quixote", which really makes you think about such matters.