Here in an 'orphan' timezone (NZ), I'm particularly prone to this, but others probably find it too: incoming messages missed by overlooking the top-line 'toolbar' icon indication of an incoming message. My Kubuntu theme doesn't make it particularly obvious, but others may have differing experience. I know many of my outgoing messages go into a black hole, probably from this.
I would venture a companion indication in the Panel icon, with a color-change (flashing?)- after all, not all mules/donkeys are the orange-brown of the icon's default color. How about a once-a-second alternation from the default to a gray (perhaps a more common color for the real-life animal) and back?