I have the step lightly perk.
Walking into the tent where the doctor sleeps at Camp Forlorn Hope, not outside the tent, just inside the entrance. There is nothing on the ground. Step, Dead.
Fast traveling to Primm, soon as I appear, boom. Reload and fast travel again, no boom.
Following the edge of the mountain area south toward Mohave Outpost, in the middle of nowhere, boom. Reload, head back the same route, nothing there.
Instant kill, no rhyme or reason for it.