Does a lecturer need to cite their teaching material?
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, &quot;It is known, even if not known to you and the other students.&quot;
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, &quot;Right out of my head unless specifically cited.&quot;  I expect your professor might answer the same way.
If the textbook were sufficient, professors could announce it on the first class, then say, &quot;I'll see you in three months for the final exam.&quot;  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 &quot;secret source&quot; 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.