Re: Swarmers and Queenless
You did not say if the drone brood was in drone cells or worker cells. If in drone cells, probably have an unmated virgin. If worker cells, then the queen did not make it back and the hive has gone laying worker. Agree with the others that a single 10 frame box is too small for a healthy nuc and the bees probably ran out of room. Swarm cells can appear anywhere on a frame, they are typically in the lower part, but not always.
Thankfully, the bees are smarter than I am. They are doing well, in spite of my efforts to help them.