Ok, this was weird. One of my splits has been limping along (intro'd mated queen) for a month now. I had two brood boxes on, bottom drawn, top about 3 frames full. No pest other than several pincher bugs running around. Good brood pattern, pollen and honey collecting just fine but not growing nearly as fast as my other splits.
It swarmed a couple of days ago to a bush about a foot off the ground about 30 feet away. It was a fist size swarm. I nuc'ed the swarm then went back to the original hive and found no swarm cells at all???
What just happened?