When looking at the CG gallery.

I still have a single one locked, have tried to search for it, but unable to find it. I must admit I didn't go fully systematic but I tried to look over the flow chart, to see what I missed.

I suspect it's a sayori ending as it's the forth counting from the end that I'm missing and that one is in between sayori endings.

Would like to hear where I made a mistake, or what I missed.

Fourth from the end is indeed a CG from one of Sayori's endings. It's from the Alter storyline where Kyou keeps Sayori's alter personality around for the entire story.