One thing I haven't seen mentioned yet: If your battery is dying way sooner than it should be, and if you have _any_ aftermarket equipment installed (radio, backup cam, alarm, remote start, trailer light module), that should always be the VERY first thing to suspect.
These things are not engineered to the same requirements as the OEM electronics, and if they are drawing 10s of milliamps while the car is off, that is certainly enough that you wouldn't notice an issue with a newer batter and a car being driven every day. But if your driving patterns change and the battery gets older, suddenly your battery is dying all the time and "nothing has changed."
This is exactly what happened to my car - I fitted a reversing sensor (this was some time ago).
About a month later the battery was flat - but we didn't drive that often so I thought it might be that. Charged it and was fine for another month or so, but then it started to happen more and more frequently. I took to disconnecting the battery each time I parked up the car.
It was only when I came to scrap the car (for unrelated issues) and went to remove the reverse sensors to use on another car that I saw a single thread of wire that was running off the reverse light, was causing a short. Ah well - lesson learnt!
These things are not engineered to the same requirements as the OEM electronics, and if they are drawing 10s of milliamps while the car is off, that is certainly enough that you wouldn't notice an issue with a newer batter and a car being driven every day. But if your driving patterns change and the battery gets older, suddenly your battery is dying all the time and "nothing has changed."