I studied up on SAR techniques last year when we prepared a 3-day familiarization course for a local youth group (sort of a merit badge thing.) There's a lot of scientific research that goes into organizing a search.

SAR personnel have to make certain assumptions. If a lost child walks less than a mile from where he/she was last seen, the searchers have an area of 3.14 sq. mi. to cover. With 100 trained searchers, this will take approximatley 10 hours to search properly. If he walks less than 2 miles, they now have an area of 12 sq. mi. to cover. If he walks up to three miles, the searchers have an area of over 28 sq.mi. to cover. Regardless of how many volunteers they have, it's impossible to cover every square inch where the kid might have gotten to, so they bring in a lot of statistical analysis and applied psychology to a search.

Most people who get lost, given a choice between going uphill or downhill, will go downhill. Therefore, those areas will be given priority. If that search turns up empty, the co-ordinators have to make a decision - did we miss him, in which case we should search the area again? Or did he do something unexpected, like travel uphill, in which case we should search in a new area.

It takes 100 trained searchers approximately 3 hours to search a single square mile. To search 28 sq. mi. in a day, you would need close to 1000 volunteers. And sending 1000 untrained volunteers out into the wilderness would be courting disaster - not only would they be likely to miss vital clues, they might inadvertantly destroy evidence that a trained searcher might be able to use, they would destroy the scent (making it impossible to use search dogs), and they might very well end up getting lost themselves.

My understanding is that the very last thing a professional or trained volunteer SAR team will do is to send a bunch of volunteers out into the woods to see what they can find, simply because the vast majority of those volunteers would be in danger themselves, and wouldn't know what to look for. (In some cases, well-meaning volunteers schooled in "leave no trace" backpacking have picked up candy wrappers thinking they were being ecologically friendly, not realizing that the "garbage" they were "cleaning up" was a vital clue to the lost person's whereabouts.)