It seems that mIRC appends that text onto the end of the titlebar. I think that it is automatically pushed off the end of the titlebar if the user-set titlebar text is long enough. In any of my maximized channel windows, I typed /titlebar x and the titlebar showed
"mIRC x - [#channel....blah blah blah..]"
I wonder if there is an option somewhere to disable this automatic text. I don't see one though, so but maybe one should be added.
-genius_at_work