Strange AirPort icon

Hi all,

I was using my 15" MacBook Pro with Wireshark to monitor some wireless traffic on my network. I had my AirPort on but not connected to any network (I was using Wireshark in promiscuous mode) when I saw my AirPort status icon in the menu bar change from the unconnected symbol to the strange symbol shown in the screenshot. Any ideas on what it means?

It was there for a few seconds and disappeared. It appears to appear and disappear randomly, hard to tell whether it disappears when I stop the monitoring.

I took a look at this knowledge base article (HT3821) but it's for 10.6, so it's a bit out of date.