I think modern cars are more likely to be 'alive' even when switched off. I had this problem with my 2005 Audi A8 which is now rarely driven. Its computer is always on, sensors alert, e.g. interrogating itself every so often to see if it's level and moving air around the suspension if it isn't. Battery lasted a mere 5yrs or so. So I hung a Halfords battery charger on the boot curry hook and plugged it into a weekly time clock to give it a boost for 4hrs every Mon,Wed &Fri. If we go away for longer than a week I disconnect the battery.