At a loss

My confusion with what's going on in Hive Cleopatra deepens.

My mother-in-law rang to say that she'd seen a swarm in her garden yesterday, presumably from the hive. I hurried out there and opened the hive. No sign of eggs (no surprises since it is a little early for a newly mated queen to be laying) and I could not find a queen. The bees seemed somewhat numerous (it's the first time I've opened this hive since I created it from a split of Hive Amidala, so it's tricky to judge anything from the quantity of bees) and they were very gentle. I searched for the empty queen cells, and only found one. One!? So who was that queen I saw on the roof at the weekend (the one I'm questioning why I put back on the hive entrance)? And why would the queen swarm yesterday? Surely not to leave no queens in the hive. I can only assume that the colony does have a queen (and therefore there is another vacated queen cell I just cannot find) and that the queen is either in the hive or was on a mating flight. It remains to be seen what's happened, but I think things may become clearer in time.