I'm not going to give examples to avoid calling anybody out, but I have come across a few instances where the search terms a user has used to find the information for their answer/comment were labeled and listed somewhere in their response. Whenever I see this I get a look-how-easy-it-was-to-find-this-information vibe, and to me it comes off as condescending. Is there a community policy/opinion regarding this behavior?
It's actually a good idea, in my opinion, especially for story identification questions.
Think about it this way: instead of it being 'oh look how easy it was to find the info', think of it as teaching the OP - and anyone else who reads it - how to find answers.
If someone asks a story-identification question, and someone answers without including the search terms that they used, that doesn't help anyone who wants to improve their Google-fu. If they do include the terms, however, they teach people about what sites to use, what keywords to use, where to put quotes...
Searching is difficult. Knowing how to use the correct search terms is hard. It's good if people include the terms - it helps people know how to find their answers better next time.
I wouldn't consider it rude
As you suggest, it is possible that the answerer's intention was to point out how easy it was to find the answer. We expect users to do some research before asking a question, and this would be a relatively non-confrontational way of suggesting that the user should do more research in the future.
It is also possible that the answerer's intention was simply to document how he or she arrived at the answer, so that others can verify it.
The user was probably educated in American modern school. As I found out to my horror, they grade you not on how well you solve the problem in Math, but on how anal you are about documenting your solution process. So, people are trained to show their steps, on pain of downvote lower grade.
Leaving that rant aside, showing how the answer was arrived at is actually a useful thing on a Q&A side. Yes, it has a slight whiff of possible condescension. The usefulness - to both OP AND to everyone reading the answer - outweighs that downside by a large margin.
Orthogonally, there is a category of answers where search terms are basically 100% required for a good answer. More specifically, any answer where either whole or part of the answer is "we don't know".
At that point, exact proof of how you arrived at "don't know" conclusion is required for the answer to be any good, which absolutely requires you to list exact efforts you put in to find the answer including exact search terms, so people can see if you missed any possible searches and try to improve.