Your webbrowser usually tries to fetch the favicon
from the "root" of
the webserver. So if your wiki is at
http://some.domain.net/wiki/index.php?Main_Page then by default the
web browser checks for
http://some.domain.net/favicon.ico
Got it in there too, no dice.
http://www.wpascanner.com
and a direct URL to it shows it in the browser.