Is it appropriate for peer-reviewer to look for possible plagiarism? I'm in the second year of a post-doc, and currently participating in my first experience as a peer-reviewer (my lab PI recommended me to the editor because he's too busy to do it right now). I noticed a section of the Methods that seems to be completely out of place. It describes statistical analysis of data that were neither generated nor reported in the manuscript, and is generally a source of confusion for me as a reviewer. So I did what I would have done grading papers in grad school, and pasted that section into a free online plagiarism detector. It came back with a 100% match for the full paragraph from an article published earlier this year. My suspicion is that this wasn't any sort of intentional plagiarism or research misconduct, but rather a case of someone using another text as a guide for how to structure that section of the text and simply forgetting to make the necessary changes, or possibly even submitting the wrong version.
I should mention, for context, that it's not uncommon to follow the methods of another paper exactly in experimental biology, but that's clearly not what happened in this case. My primary concern is that I can't properly review the reported results and conclusions without information that should be provided in this section of manuscript, which I've already discussed in my review comments. I'm just wondering if I would also be expected to point out the possibility of unintentional plagiarism, or if that's considered outside the scope of my responsibilities as a peer reviewer.
I would immediately communicate your findings to the editor and do no further reviewing until you hear back from them.
In the best case, like you say, they forgot to edit it, and can resubmit. Communicating to the editor will let them stop the other reviewers so they don't waste their time either.
In the worst case, it's poorly executed plagiarism and you would have to contact the editor anyway.
To answer the title question, of course there's no issue looking for plagiarism, and you seem familiar enough with the field to understand the implications of your results.