If the system is charging ok then look for a ""Power Drain" We see a few Alfas where the boot light has beeen removed because owners have found that it stays on and rather than find the fault they just remove the bulb. Alaso check that the glove box and ashtray lights go off.
Try leaving the car with the alarm off and see what happens.
Also....Just because the garage tested that the alternator is kicking out 13.8 - 14.2 volts doesn't mean that the diode pack in the alternator is 100%. Did they check for leak-off?
About 3 weeks ago we had a Brava in that was doing much the same thing and we discovered that the non-standard radio was drawing about 2 amps p/h even on stand-by.