Drained Battery

I have a 2000 mazda b3000 pick up 3.0L. Only 56,000 miles. I just bought a new battery because it wouldn't start. Within days it died again I switched out the alternator thinking that was the problem. It worked fine for a couple of days then same problem. I've had the battery checked and it's fine. I also had a buddy run a diagnostic check and came back clean. What kind of electrical problem should I be looking out for. I'd really appreciate any advice you could give.
Frankie.