Bees visiting former swarm site?

[tjc1]

A week ago, bees from our school hive (new package this spring) swarmed on the trunk of a small apple tree in the school garden. They were there several hours before i came and put them in a nuc and took them home (30 miles away). Today a colleague sent me a photo of a small group of bees (six  to eight) clustered on the exact same spot on the tree trunk. Are they still smelling the old queen from when the swarm was there, or is there something about the tree that attracted the swarm in the first place?

[tjc1]

After I posted this I saw that someone had just asked the same question - http://forum.beemaster.com/index.php/topic,42000.0.html -  so I have my answer, thanks!