For me it's not firing correctly, please pipe up if you're noticing it too.
Fully upgrade an Ork listening post and pay attention if it fires.
For example: I made a Biggest Listenin' and then watched a space marine scout squad walk up to within bolter range. They started firing and the missle turret on the post swiveled to face them. That's all, it did nothing afterwards.
Though in that same test above it did fire a few time at what I assume might have been vehicles. But I couldn't tell because what ever it was the missles were firing at hid back in the fog of war. All in all messing around for about 20 minutes the fully upgraded post fired maybe a total of 3 times.