All too often; when people test an alternator, they neglect to test for shorted diodes. If an alternator diode has shorted out; it will drain the battery while the car sits unused. But an alternator with this kind of damage will often still pass a voltage output test. If you have an electrical tester with an ammeter in it; one test you can make (after the battery has been fully recharged with a battery charger; not with a brief jump from another vehicle) is to disconnect the battery ground cable, and connect the ammeter between the battery negative post and the cable clamp (with the negative meter lead going to the battery post). With no electrical items turned on in the car, and no lights on; there should be less than 0.350 amps reading on the meter. If the meter reads any higher than that, there is an excessive drain from something. If the meter reading is too high; then disconnect the power cable to the alternator, and recheck the meter reading. That will tell you whether the alternator is the source of the drain. If the meter shows there is excessive drain, and it is not coming from the alternator; try removing each fuse in the fuse blocks, one at a time; and see whether the drain drops below 350ma when one fuse is removed. It removing a fuse makes the drain go down to normal; there is a shorted or defective component in that particular circuit.
If you don't have an ammeter, you can test the alternator by disconnecting the power cable to the alternator; letting the car sit for a week after that, and then see whether the battery has power. Sometimes, a new battery will be defective; so if it goes flat during storage when the alternator is disconnected, I would have the battery tested or replaced by the store where you bought it.