In the case of that "private zero day exploit", then, nobody will ever
know about it except the person that has it waiting in reserve -- and if
someone else discovers and patches the vulnerability before the exploit
is ever used, it never becomes a "public" zero day exploit. In other
words, you can always posit that there's sort of a Heisenbergian state of
potential private zero day exploitedness, but in real, practical terms
there's no zero day anything unless it's public.
The moment you have an opportunity to measure it, the waveforms collapse.