Hello. Welcome to Just Answer. Please allow me to assist you. There are usually 3 main things that cause a battery to discharge
1) Something is left on in the car (an electrical draw)
2) The alternator has an internal failure.
3) The alternator belt is loose
1) If something is left on inside the car, this causes the battery to discharge. This may or may not be noticeable. An interior light would be easy to see, a problem with the radio amplifier or some other system. This is easily checked and should have been done by the people who 'checked' the alternator. The test is called a "parasitic draw" test. This would be recommended as the next step in their diagnosis.
2) If there is a faulty diode inside the alternator, it WILL pass a charging system test. It will also fail the parasitic draw test.
3) If the alternator belt is loose, the battery is not being charged sufficiently. This would usually include a squeal noise form the engine compartment, and dim lights.
The really sounds like you either have an unnoticed electrical draw or an internal alternator issue. I highly recommend a parasitic draw test as your next move in eliminating this condition.