Of course you could have a small drain on the battery when it is standing. If the drives after starting are not too long then there will not be enough charge before the next small drain. A trickle charger will cure this, also if you have an immobilizer switch then turning that off would indicate if there is a drain or not. Otherwise try disconnecting the battery when not in use and see how it performs when reconnected, this will indicate if there is a naughty drain taking place.

Bruce Marshall
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice)
2013 M3W (Olga)