Is it permissible for a peer-reviewer to seek for probable plagiarism?

Is it permissible for a peer-reviewer (peer review, journals, writing, biology, academia) to seek for probable plagiarism?

It's not a matter of appropriateness. Going on a plagiarism hunt would be an unusual technique or concern, but flagging out plagiarism falls squarely within the purview of the peer reviewer.

If I see material I've read before, or shopworn concepts and insights presented as unique, I'll call it out as a violation of academic and ethical norms, as well as a waste of my and the publisher's time. Calling it out ensures that it is not a waste of the time of imaginary journal readers.

If I don't want to get called out for plagiarism as a researcher, I shouldn't do it. I should ensure that significant insights from others are attributed, that I am not spinning the wheel (unless I am undertaking a replication study), and that the analysis and insights I present as my own are truly my own.

A researcher's plate is already full enough without having to worry about what reviewers should or should not do.