I don't do SAR myself, but if you were to tell me to search a 1000 meter radius I'd be more concerned that the missing person might be located 1100 meters from the center coordinate rather than what datum you used to specify that center coordinate. i.e., your judgement to come up with that 1000 meter number might be more questionable than the datum you used to specify the center point. It's not like you're out geocaching looking for a very specific location, you're searching a larger area for a victim that could be almost anywhere. And when you're searching from a moving airplane high in the air, does 0.1 mile datum error really come into play?