Sometimes birds just die. I've had it happen on and off for 20 years, just as you said. Nothing wrong, then just dead at next check. I've never had it be anything the other girls got. If the bird appeared okay prior, it's not likely an illness, just something wrong with that particular bird. Pretty much all you can do is keep an eye on your flock and make sure everyone's okay over the next few days.
Please do NOT go out and buy antibiotics for your birds "as a preventative". Antibiotics are only to be used if specific symptoms are present, not as a just in case.
Good luck, hope everyone stays healthy.