It seems that Google has fixed what we used to consider a bug but was not properly a bug but simply an implementation choice.
From XMPP IM page:
5.1.3. Presence Probes
Upon receiving a presence probe from the user, the contact’s server SHOULD reply as follows:
3. Else, if the contact has no available resources, the server MUST either (1) reply to the presence probe by sending to the user the full XML of the last presence stanza of type “unavailable” received by the server from the contact, or (2) not reply at all.
obviously if we consider invisibility like a offline status we have to accept both cases. Until now we could scan invisible buddies because Google Server used two different behaviors for Offline and Invisible status.
Now the “half scan method” is no more useful. I don’t know any other method to do the same.
For now the only way I know to have information about invisible buddies is this plugin but it’s neither tested nor reliable and you have to be online when your buddies goes invisible.