My theory is that there was another BN in the tank that you hadn't seen yet.... they are very good at hiding after all and a male tends to find his "cave" and stay put till night time. So I am glad to hear you solved the mystery. I asked if you have seen the one BN you put in there because I was afraid she may have gone in the overflow along with the other one..... but that isn't the case. Just the fry decided to go their.
Congratulations on the babies and if I had to guess they will be having more soon,
Chuck