How to handle a case of unpublished plagiarism? I am a Master's student in a field of the humanities. Half a year ago I was invited to an informal colloquium of mostly doctorands and post-docs who work at the institute I'm studying at. One participant's draft for an essay was the topic of the first meeting. 

On working through that draft the evening before, I found five sentences in total that were obviously copied from online sources (blogs, wikis, etc.) without any citation. Due to stress I decided to postpone the decision on what to do with my findings and did not attend the colloquium at all.

To date, I can find no evidence online that the essay has been made available to the public and I do not know if it was ever supposed to be. Its form though is clearly that of an essay (not a sketch or an unstructured gathering of ideas), so that I can not think of any circumstance under which this would not qualify as straight-out plagiarism.

The author is still working at the institute and holding classes.

What is the professional way to handle my findings?
Not all instances of plagiarism are equally severe. So, while your assessment that this is plagiarism may be correct, it looks like at most a fairly mild case of it.

And it may not be plagiarism at all: you say the document being read was a “draft for an essay”. Well, drafts are understood to be incomplete works-in-progress, so having some missing citations (or factually incorrect statements, bad grammar, or any number of other problems we try to avoid in finished documents) is in general acceptable. If the document was explicitly marked “draft”, I don’t see how a plagiarism accusation could make sense. If it wasn’t, then it would depend on additional contextual details - I guess we’d need to know what your description of the document as a draft is based on.

A related question is, what benefit would come to the writer of the essay from passing off the quoted sentences as their own? Was the work read in that meeting graded, or was there some other material benefit they could have hoped to gain? If there was, that could make an intentional deception more likely as a possible explanation.

At the end of the day, what you’re describing sounds sufficiently mild that I don’t think you should get too alarmed that the person is “still working at the institute and holding classes”. You can complain to their adviser or some other person in charge if you are genuinely concerned, but I would expect that this person would, at most, be told to be more careful in the future, but would not suffer (and do not sound like they deserve to suffer) any more severe career consequences.