So I looked at the code to answer your question.
Thanks! I searched the code myself today with no luck. I postoponed it for the weekend but now you have resolved this, I'll just have to stick to something else to do :)
My answer is more or less an educated guess. If you want to look around to confirm or for fun, PL_sv_no is the SV returned when a builtin wants to return something false. An interesting puzzle would be to see if you can manage to lower the refcount and display it at the lower refcount using Devel::Peek's Dump.