You're right, we're wrong.
Looks like Yahoo! has changed something and doesn't know that people can check up on them if they're invisible or not.
I did some monitoring on the packet that sends out after you check the ID and by the looks of it it's just a packet that waits for a response from the Yahoo! User
The packet it sent to me was.
PHP Code:
4persiangap_robo_45dosed2062
Maybe if this stays permanent, Brock or Adam could add onto YTK for a stealth-checking feature?
