Things such as interior lights that do not turn off, radio station presets, the clock, engine computers and body-control modules all drain a car's battery even when the car is off. Other items like proximity keys, aftermarket stereos and amps and alarm systems can also draw excess voltage that drains the car's battery.
The cause of a drained battery can be because something is using more battery power than it should, such as an electrical malfunction that keeps the glove box light on all the time. Other things, such as leaving the car off but listening to the radio in the car or using the overhead interior lights when the car is turned off, can cause the battery to drain quickly.
If the battery is draining, and it is not due to user error, such as forgetting to turn the headlights off after parking in the evening, the fuses should be tested with a multimeter to see exactly where the battery drain is coming from so that it can be fixed by a professional. When testing the fuses, the battery must be fully charged, the car doors must be shut and any charging cables must be unplugged from the lighter.