After watching the fire knife Easter egg, I remembered about the time I was trying to get the achievement by completing Evacuation under 18 mins (which I did after 8 tries). I would pass a wide open space with a small hill before meeting the prometheans shooting me on their very own warthog. I would remember hearing the grunt birthday cheer, but I just thought it meant I was really near the skull, so I never bothered until I finally watched the flaming knofe Easter egg video. Does someone have info on this? Too lazy to check 
I think I ran over the Easter egg trigger with the warthog and the grunt birthday cheer turned on
To get the skull on that mission you need to destroy 5 street cones, one of which is on the bridge. When you shoot it it makes the grunt birthday sound.
My guess is that you destroyed it by accident and heard the sound.