Have you ever encountered such a situation (e.g., Reddit, or even HN)?
N.B. The staggeringly elaborate Cicada 3301 puzzle [1][2] does require deciphering steganographic messages, but they aren't posted in public channels.
[1] https://www.youtube.com/watch?v=RatbYqc0-jE
[2] https://uncovering-cicada.fandom.com/wiki/Uncovering_Cicada_Wiki
Typically this happens on websites that look like junk/ad websites to you and me. Also sometimes they will look like the ads you see on parked domains.
There will be hidden words, or perhaps you will need to know how to adjust the CSS in chrome developer tools. Often times it is a small URI fragment you need to append to the URL, or a specific place to click every time.
Again I want to emphasize how common this is, and the “dark web” is not used nearly as much in terms of overall volume by such groups.
Owlhousing.com was until recently and still probably is one such site. It looked like some junk casino ad website, but scroll down for a very lengthy period and you would find...sophisticated electronic Microscopes for sale out of Hong Kong. Now it is some car website.
The best way to find these websites is to use Spyse.com. Using the “prefix” search operator enter something like “000000000” or “aaaaaaaaa”. Hundreds if not thousands of sites show up. No real person would take out these domains. Start with them.
Edit: Improved wording
I was reading a thread between two users that seemed totally normal until a user posted a screenshot of their comments. It showed a second conversation between them that was happening in secret. If memory serves the person who posted the screenshot said it was an extension (possibly RES) where you view certain tags and allows you to see the extra comments based on the tags.
After seeing that I couldn't help but wonder how many secret conversations were happening out in the open on Reddit.