recent era cars leave many things powered up after the key is turned off ( memories, alarms, door locks etc )
a current drain audit can be performed, which will show how much current is being consumed with the " key off " this will show actual usage or a fault in wiring, either way you will find how much current is draining from the battery
easy fix is to use a battery maintainer...for periods longer than a few days also it will increase useful life of battery
replace older battery, as batteries age their capacity to supply power declines
change the type of battery construction or purpose of use... starting batteries are designed to supply a lot of power for only a few seconds, any drain will allow these battery to go flat quickly. replace with a battery labeled RV /Marine/ deep cycle. These batteries are designed to deliver power over an extended period of time. For accessory power this is measured in days of extra power vs a std starting type of battery. This number is measured in ampere hours /AH or called reserve capacity when checking specifications. Current batteries are much more powerful than in the past, allowing the RV / marine / deep cycle type of battery to be used as starting batteries. Typical reserve capacity of a starting battery will be somewhere between 50 and 80 ah or less, with deep cycle style, this number can be in the mid 100's for the physically smaller to over 200 ah for larger size batteries, easily doubling or tripling the time to power accessories before failing to provide starting power. Don't be impressed by numbers expressed by ratings labeled as cold cranking amps, this is power measured in seconds of service, and has no bearing on delivering power over time. Best to become informed about what a particular type of battery can or cannot do, there is more to a battery than the 12volts marked on it. All batteries are limited by their reserve capacity measured in Ah, think of it like the size of the fuel tank, capacity determines how far it. can go.