Is it considered plagiarism for a professor to use uncited sources in teaching materials?

Plagiarism involves dishonesty: taking someone else's work and leading others to believe it is your own.

In the case of unpublished teaching materials, I don't think it's assumed that all the examples, etc., a professor uses are original unless an outside source is cited. So I wouldn't strictly consider this plagiarism (though citing outside sources is definitely preferable to not citing them).

It's possible that in your academic culture it is assumed that all of a professor's materials are original, in which case, the above would not apply.

Note that I'm not in the US, but in my experience generally it's never implied that all the teaching material is original. There often is a hereditary element too, or the department has common material they share.

Also consider that the only sensitive material in most courses could be copyrighted images. There's no copyright on Newton's law. Plagiarism would be saying that you discovered Newton's law, not copying another teachers' material.

I've noticed that some people put a password on all their PDFs and say it's because of the material they put in it which they don't want to show up in google images. Often, I've noticed that it's a mix of stuff they wrote with images from those PPTs you can get from the textbook minisites, given free of charge by the publishers so that teachers can use the same pictures that are also in the books. I haven't read the terms and conditions the publishers put on those slide shows but maybe they don't want the images to be republished without a full citation of the source, hence the practice to keep access restricted to the participants.

The simple answer is no.

Usually, courses are being taught with the same material every semester which is "inherited" from one lecturer to the next one and usually begins with some textbook so that's not unusual - no one assumes that it is an original material.

More than that, if a lecturer is using exercises from some textbook in class he might also use them for homework sheets and then he would prefer not to disclose their origin in case there are also answers in the textbook.