Some times you catch a swarm and go to do an inspection 4-5 days later and there are no eggs, but there is a queen.
I think this is a young queen that has hatched in a swarmed hive, goes out on a mating or orientation flight and the bees are in such a swarm mode, a queen is leaving then we will go also. Or they are in such swarm mode they just swarm anyway with the new queen. Why they do this is a mystery as it must be the death of the original hive unless there was a late formed QC that was not torn down.
We do have some queenless hives after Spring that I suspect are hives that the new queen does not get back or are second swarmers.