Yes, if introspection is to be likened to a sense it must detect something. The question is whether phenomenal properties are a part of this something. The ability to detect internal states alone does not require phenomenal properties in my opinion as even a computer can do it.

As I said in the OP I am skeptical of phenomenal properties, my main point is that skepticism should not be equated with Illusionism. Skepticism of phenomenal properties has been my position for a long time, it does not mean I am in an emotional state of doubt, confusion etc.

"1. Something that deceives by producing a false or misleading impression of reality.
2. The state or condition of being deceived; misapprehension."
See the "space" that these definitions open up?

In the second case, although there are different theories of "illusionism" in philosophy, I think that the most common and what I personally came to know about is one that has to do with the nature of consciousness. A view belonging to "eliminative materialism", which considers and describes phenomenal consciousness as an illusion. — Alkis Piskas

I am thinking of illusion in that first sense, as a "deceiving appearance", and yes by Illusionism I was referring to the eliminative theory or at least broadly the idea of phenomenal properties being illusory (and thus presumably targets of elimination). I think that the "illusion" is taken to be sensory, the sense-introspection analogy is very important to the theory and I think illustrated well by a passage in SEP just below what you quoted:

"Illusionism claims that introspection involves something analogous to ordinary sensory illusions; just as our perceptual systems can yield states that radically misrepresent the nature of the outer world, so too, introspection yields representations that substantially misrepresent the actual nature of our inner experience."

What I want is to single out the process prior to anything resembling the generation of a proposition (i.e. something that can be true/false), perhaps calling this "perception" is an abuse of terminology. There is a causal connection between me seeing a red apple coming to believe the proposition "that's a red apple", but the apple does not speak out to me and tell me about itself, I am the one creating that proposition. So if the proposition is false, I have only myself to blame.

The camera analogy does the job insofar as the camera perceives/records/whatever-you-want-to-call-it without making judgments whose truth-value could be subject to evaluation.

While it may be that it's not human nature to perceive without also interpreting, I think the two are distinct. I would say a camera is an example of perception without interpretation in the sense I mean.