There is an overrun pump for the cooling system which can be heard when you switch the engine and electrics off. If this is not shutting down when it should it could be slowly draining your battery. Is the fault more noticeable if you leave the car standing? ie not driven every day. This pump can be found back right of engine compartment. Check this and if not I would suggest a good battery check is needed. had the same problem had to replace my battery