Hiding an SSID only removes the name from Beacon frames that are broadcast. The SSID is still broadcasting beacon frames so it is discoverable and the name is still in Probe/Response frames. So a device passively listening can catch a Hidden SSID when it talks to a client.
So basically hiding your SSID is just giving you a false sense of security because it hides it from people who don't know any better but anyone who knows anything about WiFi security can find it.
Interesting. I've never seen it show up before. EDIT: I probably just wasn't looking for it.