The checkmark seems to be gone now. I threw some basic MSNP18 login requests at it anyway, but was rejected as per normal.
Hard to say but I suspect it was just a ‘false positive’.
Yeah you are right, its gone again lol. 
Hyperz (from IRC) earlier today that four more had checkmarks at one point, bay servers again, then were gone a half hour later. I’ve started my own scanner specifically looking for this that runs more frequently and maybe that will validate what’s happening, assuming it happens again.
Yeah its still a bit strange.
Unless Microsoft has still some planes with the msnp18 protocol.
But i highly doubt that.
Good job to set a scanner for it, to see whats happenening.