(The later the model year, the more gadgetry now (burglar alarms from factory or 'listening for a key-clicker' to unlock, etc.) Got a non-toy volt-amps-resistance meter? 'VOM' that is.

Simple test re all the 'smart'-gadgetry added:
just disconnect (either battery cable) and put an ammeter in series. Ohm's Law tells you whether the nominal drain is *normal or unreasonably high; pinning that down is the tedious task of disconnecting things, as can turn into a nasty Bug-hunt.) A sick transistor or a capacitor can account for many of these drains.

* Under 30, 40 mA (milliamps) is upper limit for normal. You can do the arithmetic -vs- battery amp-hour rating; it's just algebra E=IR right? At "a tenth of an amp" say, you can see that it's not many days before No-go.

(Cable back on terminal) Couldn't hurt to note voltage at low up to medium RPM, thus eliminate alternator or its regulator not recharging well.. Lots of batteries are replaced via guessing rather than a few minutes sleuthing.
Over here that can kill most of a C-Note.

Luck