This question already has an answer here:
We heard many news reports of many pardons Obama made. But is it possible Obama could have secretly made other pardons as well?
For example, assume Obama pardoned, say, Hillary Clinton, for example: Obama gave Hillary an official, legally binding document with a pardon on it; only Obama did this secretly and neither Obama nor Hillary told anyone about it. Now let's say years later, Hillary gets prosecuted for a "crime against the United States?"
Could Hillary pull out her pardon and play it like a trump card?
Or, more to the point, do we have any way of knowing for certain that Obama did not make any secret pre-emptive pardons?