I have seen this little icon appear against several names on the Online Members list for quite a while now.
Exhibit A --->

Okay, I know it allows you a quick link to send that person a PM, but it seems to appear randomly against some names.
I left myself logged in from about 6pm and it wasn't there then. When I came back and refreshed the page about 3 hours later it had appeared beside my name and also a couple of others.

I'll probably get a lot of sarcasm about this, but I'm just curious why it doesn't appear every time. Is it just another system quirk that runs an hour later than normal, just like the clock?

See Weird Times in Boardania

It mean you were using the IM but you are not there anymore.

We have IM ?

edit: Forgot to say thanks.