Sooooo we got a problem.. one Silent Fang at The Grottos on Cosmodrome instead of three.
I found a discussion on reddit with a temporary solution: Kill the one silent fang, go to orbit, then relaunch patrol on cosmodrome and go kill the one again and repeat until finished.
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Aren't there three Silent Fang in the mission where you go to this are..? I can't remember exactly, but I have a feeling there are more in that mission.