Battery problem indicator keeps coming on

My battery problem indicator keeps coming on. The electrical system has been load tested at the GM dealer's shop. The alternator and battery both test fine. These tests were done when the light was not on. The battery posts were dirty, so they were hoping that would take care of the problem. After a week, the light came on again. The shop only wanted me to bring the car back if I could bring it back while the light was on. Before I could get it there, the light went off again. With all the equipment they have, why can't they determine the problem with the light off? Further, does anyone have a good idea of what the problem could be, how might this be determined with certainty, and what would it take to fix?

Thanks.:banghead:

maybe a bad ground or a slipt in the wire...and it only messes up at certain times thats why they can read it on teh computer