This happens often.  Bees swarmed, there was no queen with the swarm,
the swarm returns to the hive.  This always happens with clipped queens.
Swarm leaves with queen, queen can't fly and falls to the ground and is
lost, swarm settles without queen, swarm senses no queen and returns to
the hive where there are plenty of developing queens in cells.  A few
days later, when a virgin queen emerges, swarm and new queen leaves.  It
is as sure a bet as one gets in beekeeping that your hive will
successfully swarm within a few days unless you take steps to keep the
bees at home.  Corrective actions can include Demaree Swarm Control,
making splits, Snelgrove swarm control, all of which basically throw
swarms from the original hive but successfully keep your bees at home
rather than losing them to the wild. All methods are covered in the
archives or can be googled for a full description.

"If a colony disposed to swarm should be blown up with dynamite, it
would probably not swarm again, but its usefulness as a honey-gathering
institution would be somewhat impaired."  C.C. Miller

Aaron Morris - plugging http://bees.library.cornell.edu/

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