In reading about engineers on halopedia. I saw this that came of interest to me: "they (engineers) will utter a high-pitched keening sound whenever a Forerunner artifact is under any sort of threat." Since humans are descendants of forerunners and Mombasa has buildings, artifacts, etc of forerunners maybe you do have to either shoot or grenade their artifacts to trigger something. Maybe hurting the already dead elites (just guessing). On a side note, do the pillars in the stonehenge produce musical tone when being shot at, grenade thrown at it, or melee (sorry if I misspelled it).