Whilst cars are stood unused there are small drains on the system , more so now in modern cars than in the older pre 2000 cars , alarms system etc etc , your car goes into a sleep mode and is awakened by the key ( either pushing the unlock or it senses the key and unlocks etc etc, modern cars are also power hungry , electric power steering systems and lots more electrical systems with demands etc etc
You can get a local technician to drop test the battery and check condition , then check battery drain with an ammeter , a solar panel on your garage or house roof ( or the floor mounted large fan type variety can be used to keep your car battery topped up , or invest in a trickle charger for times when your car is out of use for long periods