Jonnyboy,
If the alternator field circuit breaker fails, it will take the alternator off line, and there might not be an "ALT" annuciator iluminated. This happened to me in a Cessna (same system probably) and I flew around for who knows how long on only the battery. The day VFR discharge really is not enough to notice on a 60 amp ammeter. Finally I noticed because the battery went flat just as I got the flaps all the way down for landing at another airport. I Aborted the landing and flew home that way! It took a while to troubleshoot... (alternator was fine though)
Cheers, Pilot DAR