It might be just bad luck. Was it genuine Bosch/Valeo alternator or aftermarket or rebuilt unit, is it still under warranty? I suppose I can see that being in storage could cause corrosion buildup making bad connection for brushes or getting them stuck stopping alternator from working properly.
It's very unlike y for alternator to have terminal failure. It's normally voltage regulator that fails or brushes wear down or it gets noisy because of worn bearings.
I recently had alternator failure on our A6. New genuine alternator would have set me back £400-700 depending which one I chose as there are several Bosch and Valeo units that can be used and are interchangeable. Cheap aftermarket one would be starting at £130 but I have seen too many cheap units fail within 2 years, many even in less than year. Apart from not wanting to spend money I also did not want to take the front off the car to get belt off. Old unit was Valeo and voltage regulators are hard to find for these, one from dealer would cost £70, that's still too much, I found one on German e-bay for around £20 but before placing the order started to investigate. I took the voltage regulator off without taking alternator off, brushes were worn quite a bit and a bit sticky not moving freely also couple solder joints looked dry. Commutator looked fine, it did have some wear but probably around 50-60% life still left in it. And judging by the date stamps from year 2000 it is still original alternator done over 170k miles. So I gave it a good clean, re-soldered all the joints ant put in new set of brushes from e-bay for £3.55
http://www.ebay.co.uk/itm/1809397748...84.m1497.l2649 You can get them for £3 with economy delivery but I chose the ones with first class delivery. All fixed and working fine and hopefully will last another 150k+ miles unless bearings wear out. All for the cost of £3.55 plus 2x 1 hour of my time.