Volage drainage?

My Renault Master has had a brand new battery and alternator but once it is not driven for 3 days the battery is flat, I have had nothing new fitted to the van for a few years so it's nothing I have caused. The battery has been replaced even though the old one wasn't that old but it hasn't solved the problem.
Has anyone got any ideas why it is draining, it is on a 52 plate.