I’ve had two friends with virtually new German brand non hybrid cars that have had battery problems. Both drove long distances on the motorway to an airport. One was away for 7 days, the other 10. Both had flat batteries on return. RAC person said it is getting more common and attends Heathrow everyday with the same scenario. Supposedly some modern car alarm systems have a high power usage so can cause drain over a relatively short space in time. As both of them are regular fliers, I advised them to change to the highest spec heavy duty battery they could buy. Neither have had any problems since.