Honey bee colony populations can increase so rapidly just before swarming that it may not be apparent that a hive has swarmed just by population or entrance activity – although usually both are reduced somewhat. Upon inspection of such a hive you will usually find queen cells along the bottoms of combs (swarm cells) – probably opened where the queen(s) have emerged. There will not usually be a lot of brood in the hive right at the time of swarming because the queen typically runs out – or nearly out – of room to lay eggs in during the run up to swarming. You may find several recently emerged brood cells – which may be filled with nectar. But the main sign of a hive which has recently swarmed are opened swarm cells.
So, what if anything do you do? If there are a lot of unopened swarm cells scattered through the hive you should remove most of them from the hive one way or another. A hive which contains many widely scattered swarm cells is likely to issue multiple virgin swarms which can seriously deplete the hive population.
You can destroy most of them leaving only a few on one or two adjacent frames – so that the first virgin to emerge will kill the remaining ones.
Or you can make up mating nucs with some of them by putting a frame with a queen cell and a frame of stores along with all of the clinging bees into a nucleus hive – good insurance in case another hive becomes queenless. Shake in enough extra bees to cover the frames, and of course fill the box up with frames of comb or foundation.
Either way, don’t leave a big strong hive full of unopened swarm cells.
Once you have taken care of that – if it was called for – be sure to inspect the hive every week to monitor it’s condition and ensure that it is not going to become or remain queenless. If at any time you don’t see eggs or very young larva in the hive then give it a frame of young open brood and eggs from another hive. If on your next inspection there are queen cells on that frame then the hive is queenless. You can either let them make a queen from the cells they have started or you can go ahead and give them a queen if you have one.
If you let them make a queen then you need to continue to monitor the hive to make sure that it doesn’t start to get weak – if it does you need to reduce it’s volume by removing frames or giving frames of stores to other hives. If it does not have a laying queen by a month after you discover it is queenless the hive will be in danger of being over run by hive beetles, wax moths, robber bees – and ultimately collapsing. But that won’t happen if you do your inspections and take timely action to remedy queenlessness.