Last Sunday I replaced the battery in mrs. Maser’s Toyota after only 4 years. Which invisible light has she been leaving on again? 
On our Toyota Camry, it is the rear seat reading lights that the kids can leave on unnoticed that will quickly drain a battery. All lights will stay on for about 30 seconds after you leave the car. The car will automatically turn off the headlights and other lights but will not turn off the reading lights.
I replaced my car battery when it started to struggle, only to realise quickly that it was actually the Alternator that was the problem.
I’m no mechanic but as I understand it all the car battery should do is provide the grunt to power the starter motor to actually turn the engine over. After that, while the car is running the alternator should be providing all the power needed to run the car, as well as topping up the battery.
Of course, sitting in the car with the radio/lights/heater etc on when the motor isn’t running is a sure fire way to drain the battery.
An easy way to test if it’s the battery or alternator is to do this.
Start car. While car is running, disconnect battery.
If car keeps running, battery is the problem.
If car conks out it’s your alternator.
BTW, I wouldn’t suggest doing it for long or in a new/expensive car. It can cause issues.
Do yourself a favor and DO NOT do this. Ever. At all. It’s possible to damage components like the alternator and the computer(s). This technique had some merit 50+ years ago on cars that had DC generators. On cars with alternators* it can lead to expensive problems.
*There has been a trend over the last few years for car manufacturers to refer to alternators as “generators.” It’s fine to use that term, but be aware that all modern cars have alternators, which produce AC current which is rectified to DC, and are absolutely NOT DC generators such as were used decades ago.