The Vanishing Messenger

My 2 year old has a knack for find hotkeys that I didnt even know existed,
and for messenger hes found one that hides it completely, I know its still
functioning, but I cant make it come up, its even hidden the icon in the
task bar, and its not available for alt-tab... This must be a hidden
Hide-it-the-boss-is-coming key, any body know what it is, he keeps doing it
ands its annoying?

Thanks in advance


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