Years ago I was at a local match hosted by a club run by guys I knew very well. After the match was over, I was standing by the safe area when a competitor removed his gun from his holster. He racked the slide and a round popped out. If he had just finished a stage, he certainly would have had to unload and show clear. So how could this have happened? I don't know the answer to this question, but clearly something happened that should not have.
As the RO it is their responsibility to make sure the range is really clear before everyone heads downrange to paste targets and set steel. If the RO isn't comfortable with what he/she just saw, stop the shooter and make them go through it again.