there is a possibility if he went to a and e in the evening say, he was still there waiting to be seen in the night - to me that would count as being kept in overnight
One morning I got up and realised my son had not been home all night. My husband rang his place of work, as we had assumed he maybe had some overtime. It turned out that he had got into a fight and been hit in the eye (at work and his fault) and had been in the Eye Hospital ever since and no-one had bothered to inform us. Not the place where he worked nor the Hospital. So it can happen.