A failed diode in an alternator can discharge a battery.
A stuck relay can discharge a battery, as can a failed sensor, a switch allowing the underhood or trunk light to stay on, etc.
Easiest non-spendy way to BEGIN to determine where the draw is (if you have a test light), and what I'd do to start diagnosing the problem if the car was in my own shop:
Close the car door (to eliminate getting a false-positive from the interior lights).
Disconnect the positive cable from the battery.
Connect the test-light between the cable and the battery terminal.
IF there is a current draw somewhere, the test light will light up.
Open the fuse box cover.
Remove one fuse at a time, look to see if the light goes out, then put the fuse back.
The fuse that turns the light out will indicate the circuit, or group of circuits, where your problem is...assuming it's something straightforward.
I' you can't find the general location that way, testing the alternator (by someone who's competent) is the next step.