Is Camp Searchlight a location one MUST discover independently, or is there a quest or an NPC conversation that marks it on the map for you?

As far as I know you have to discover it independently, although I don't see how that is a problem.

well, two roads lead to Fortification Hill (place you HAVE to go to unless you're following the NCR's quest path) , and one of the two roads is through Camp Searchlight.