My lecturer uses powerpoints as a teaching material. During the first class she told me that we are using a book as resource for our class. But one day I realised that some parts are not from the book. So the question is: should she provide the reference she use? IMO, she should tell us what she is basing her teaching on.
As with writing, anything presented in a class without citation should be either the work of the presenter or common knowledge. Your teachers are supposedly experts in the fields they teach, and so should have amassed quite a lot of the common knowledge in their fields. For example, I'd happily describe the Bell-LaPadula model and its applications, mentioning Bell and LaPadula by naming the model, but without further citation.
For those courses I taught that had a textbook, I organized classes around the textbook, but certainly presented material not contained therein, often without citation, because, as Buffy has written in another answer, "It is known, even if not known to you and the other students."
If I had used only the textbook publisher's slides, I'd have felt I was cheating my students. If a student had asked me the source of material in the lectures but not in the textbook, I'd have said, "Right out of my head unless specifically cited." I expect your professor might answer the same way.
If the textbook were sufficient, professors could announce it on the first class, then say, "I'll see you in three months for the final exam." The role of the professor is to explain, elaborate, and expand. The latter two necessarily go beyond the content of the textbook.
With all of that said, I'm not sure I understand the goal of your question. Do you believe there's a "secret source" that's somehow being hidden from you? I doubt it. More likely, there are many sources, accreted over years of study by the professor.
If you're looking for material to supplement the textbook, ask. Professors want you to learn and if there are accessible supplemental resources, they should tell students about them.
For a few reasons, a lecturer should let students know the source of their teaching materials. But not everyone will and some might actually have reasons not to if they are trying to guide students toward some things but away from others. That should be rare, I think.
But the issue can be a bit subtle. Is the use of materials intended for teaching without citation a form of plagiarism? Is it a form of improper "publishing"? People will answer differently.
Plagiarism is about ideas, not their explicit expression. If the instructor is teaching well known things (common knowledge) then there is no implication that the things taught are the ideas of the instructor. Hence it is hard to claim it is plagiarism. It is known, even if not known to you and the other students.
Publishing the work of others without citation is an offense, but against copyright. But is teaching a form of "publishing"? And is there an implicit license for teaching materials to be used in teaching? It isn't always obvious.
So, I find the legal and moral issues a bit cloudy and would, therefore, defer on those issues to faculty in most cases. Not all. Most. But since the issues are (IMO) on the borderline, it is probably better to cite them than not to cite them unless there is some firm reason not to.
But, there is an educational reason for making the source(s) of materials available to students. Especially conscientious students will, perhaps, occasionally, want to follow up with those sources and thus enhance their educations. I applaud that when it can be made to happen.
But there is an action item here. You can, perhaps, ask her for her sources. I suspect that you will likely get them. If she refuses, you can validly ask why. But to save yourself from some grief, ask in a non-confrontational way. Perhaps she is happy to share them. In fact, if you have already been given a list of resources for the class, as some instructors provide, you might even find the materials there.
And, of course, some slides and such are produced by the instructor herself, based on her knowledge, though it may be common knowledge, the source of which she has long forgotten.
For some materials used in advanced courses, there is probably a stronger case to be made that not citing them is a violation of plagiarism standards, copyright, or both. I've assumed above that this is not the situation here.