I'm using xfce4-panel 4.11.0 in Ubuntu 14.04. I found that is I have a program opened in Xfce and I push my left mouse button down when it is on any program tab on the separator, the icon of the Whisker menu (like a rat face) disappears and stays disappeared until I move my mouse over the Whisker menu icon.
First of all, this is a bug in whisker-menu and secondly, I'm pretty sure it has been fixed meanwhile. If not, feel free to report it where it belongs: https://github.com/gottcode/xfce4-whiskermenu-plugin