I was called to catch a swarm on the canopy of a gas station a few years ago. When I got there it was a small cluster and after catching them no queen. I asked the manager of the station where he thought they had come from and he told me a truck load of bees had come in and fueled up. So I assumed these bees were stragglers from the truck. I took them home and shook them out in front of one of my hives.