I remember i saw once a comment about Messenger Discovery and it has an option to see those people who are "probably" appearing offline, don't believe me cause i've tried to use that program and my msn crashed so i keep Msg Plus! and the scripts i love so much xD and forgot about trying to check who's hidden hahaha
But it seems to be the answer to your question, if that program really give people a clue about your "appearing offline" status... then for sure he just has to give it a try when he see that clue
and has more probability to be right
See ya
EDIT: hey guys i decided to check just cause im too curious xD and this is what the program's website says:
New Alerts
* when a contact is appearing offline
* when a contact changed their personal message
* when a contact changes what they are listening to
You can check it out on the changelog... hmm and people says is magic xD (but i still don't believe it works 100%)
That's all ^^