That seems like an unreasonable expectation for your users. As you’ve just described, it’s really hard to find unlisted topics. Rather than unlisting them, I’d recommend creating a subcategory, perhaps one hidden from the main page, to hold the errant topics.
To find the ones you’re looking for now, you might check out the Data Explorer plugin. There’s a “hidden” field in the post table. I believe that if you search for it, you’ll find your unlisted topics.