The correct answer is D) The search target is sighted and then lost. A sector search is typically conducted when the search target has been sighted and then lost, as this technique allows the search area to be systematically covered to try and reacquire the target. This is in contrast to other search patterns that may be more appropriate when the initial search location is known, such as a sweep search. The other answer choices do not directly necessitate a sector search, as the availability of aircraft or multiple vessels, or the detectability of the target, do not inherently require the use of a sector search pattern.