i believe the command ISON is used here. IRC clients like mIRC use the ISON command for notifys. Any user can do this, theres no need to be an IRCop. So any IRCx server (IRCd also not sure) have this command and is useable by all users.
Glad your finding good use for it Morph
Also, Ozzy i followed a tutorial to make the switchbar and added other stuff like Windows etc, and it wasn't made for CUSA.
Edit: mIRC sends the ISON command when you first connect, if a user is online the server sends back the nicks online using the raw 303.