I don't know how they got there but it's obvious that babies from your mbuna tank got into your con tank. They didn't just miraculously appear. If you didn't have mbuna at all I'd be stumped but you have the same adult zebra in your mbuna tank as the babies in the con tank. How they got from the adult tank to the con tank is the puzzle and frankly I wouldn't put too much thought into it. If you can't recall well then it just happened. Moved decor? Shared a filter or net? I've personally had female mbuna spit a mouthfull of fry just from fright of seeing the net hit the water. What about your siphon hose? All possibilities.
As for not noticing spawning behavior or aggression due to spawning behavior, it's not surprising. With 40 fish crammed into a 55 gallon tank I can't imagine your fish could make a target out of anyone else. Dominant males don't target ripe females, they simply lure them to the breeding site and in a tank packed so full the act of finding a spawning site, perfecting it, defending it, luring a female into it, spawning in it and moving on may only take a few hours. This is how even a subdominant male can sneak himself a bit of action in a tank with a dominant male defending a permanent territory.