I’m quite sure there is no such case. I would bet it must be something automatic process happening on background, and I can’t undestand that either.
On my forum anon-posting happends almost never and even then it was by regular users. There is small chance I’ve deleted some old and non-relevant topics, but I can’t remember there would be anons either. And yes, my forum is so low traffic that I can rely my memory over SQL 
I had one user who didn`t confirm email for some reasons and system killed his account some days ago. All he did was giving likes, nothing else. And now I have a feeling (sorry I can’t give more solid info) that error started popping up at same time.