It usually means that he can perform all actions to some of his contacts, like talking to them, while the rest thinks that he is not at his computer.

Buddy Spy is a software program for Yahoo! Messenger. It can detect if a buddy is indeed offline or invisible. The software can also check if the user is currently broadcasting with his webcam, if he is in any chat room and if he is on an alias profile or default profile.

Buddy Spy requires the creation of a new Yahoo! Messenger account, usage of the default account would interfere with Yahoo! Messenger and cause errors. After the account has been created the user can use various scans to detect if a buddy is online but invisible.

There is however a way to find out if your Yahoo contact or buddy is really offline or only faking that he is offline. It basically lets you detect if they use the invisible mode in Yahoo Messenger.

The easiest way is to use a script like that offered by Xeeber. Just enter the Yahoo ID of your contact or buddy and wait for the script to display the result. You will find out if he is using the invisible mode or really offline.