What @delirious_owl@discuss.online seemed to be implying is that direct messages on Mastodon should be considered “public” rather than “private”.
I’m assuming that’s along the same lines of how Lemmy users generally think that their upvotes/downvotes are private when in reality, if you know how to look for them, you can see them.
Most of that just looks like carpet shading, where rubbing the carpet one way looks darker compared to rubbing it the other way. http://carpetswalltowall.com/shading-footprints-pooling/
However, the brown stain on the far right definitely stands out.