Listed in my quests, under 'miscellaneous', is Defend Graygarden. I fast traveled there to find only one bad guy present and they were quickly dispatched, but the quest didn't update or become complete.
I'm playing on PC so all console options are available to me. Attempted, but with no success, I tried
disable/enable on the one corpse,
resurrect no less than ten times, and
killall - nojoy. The first time this happened, I just had to scout a (very) wide perimeter to find the baddies; this time, no such luck.