battry losing 2volts over night ?

I have a RENAULT Scenic 1.9 dCi 2004 Iv had the car about 4 month the car was not used for about 3 weeks over charismas and the battery was to flat to start it I charged the battery and tested it over the next 3 days and all looked ok battery holding change of 12.6 volts off the car and alternator working on the car at about 14v but two days later battery again to low to start car so I changed battery for the one I have in my van and retested battery from car and all ok. Anyway this is now happening every day or so. So when I park up at night all ok next day to low to start I have cheeked all lights inside car and that everything is of but it is still losing about 2 volts over night something is darning the battery but what.. ???
Can anyone help