swarms landing in the same spot are not so uncommon. i always tell people to watch for more. i have picked up swarms and had people tell me that there have been swarms in that tree/whatever, before.
Ok, so that was an abscond, 'eh Mick? You better tighten up the hatches on your castles. Have your bees and Bud's been visiting? You know how Bud's bees are, swarm out on ya in a second!...JP