You may check this Edit: or you may check this http://forums.digitalpoint.com/showthread.php?t=705115&highlight=invisible+yahoo
Check a wonderful Yahoo Messenger Invisible Detector located at www.imvisible.info It's easy to use it, 100% accuracy, works perfectly with YM9, web messengers and one issue that a lot of detector doesn't have and IMvisible has is that it detect yahoo messenger users that use firewalls Take a look and give me a feedback