Maintenance free batteries have a habit of just failing. Once it's failed the first time it will usually continue to do so.
Mrs Statler had a Pathfinder battery fail after 2 years, fortunately replaced under warranty. It failed two times on us before we had to get it replaced. Each time I charged it up and it lasted a few weeks. Usually this happens during extreme cold weather.
RAC man on the TV this morning said much the same, weak batteries don't like the cold.
Any local vehicle electrical specialist should be able to confirm it's condition for you.