unless you got feral hives close by i bet they are comming out of your hives. i have seen hives swarm and you couldnt tell they swarmed by looking at the numbers but remember it takes 2 to 3 weeks for a new queen to start laying that is when the hive will look weak. if you only have a few hives and you notice a swarm look between the to brood boxes and see if you see swarm cells