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In the case of unpublished teaching materials, I don't think it's assumed that all the examples, etc., a professor uses are original unless an outside source is cited.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.