Split a hot hive last weekend with the plan to requeen using a frame of brood from a donar hive. The half of the hive which was moved didn't show a lot of activity but then got real active Monday. Thought that it was getting robbed out but the bees just were not acting like robbers, they were calm and weren't diving in and out of the split box and the bees weren't fighting on the landing board. Took a good look at the hive today, didn't open it up but just watched it for a good 1/4 hour and oops looks like I picked up a swarm, not planned, not needed but looks like the first swarm of the year. :-D Don't know if the swarm came from my yard or not which is another mystery.
I had the same thing happen two years ago while culling out old frames, left a box in the bee yard loaded up with old frames and had a swarm move in when I was taking a break. Gotta love it, you never know what will happen or when.