As Tripod noted, you now know where Kevin is not, so you have reduced the search area. This increases the probability of him being found somewhere in the remaining search area. If all of the trails have been searched, that is additional area that can be checked off, further reducing the search area. A very fit individual can walk up to 30 miles in a day on a flat trail with no obstructions. Hilly terrain, wearing a pack and hiking on rough terrain reduced that significantly. If you take the last known spot (PLS) Kevin was seen at, put a dot on the topo map. Figure a max of 20 miles, using the scale of the map. Take a compass, set it for the 20 mile distance, put the point on the PLS and draw a circle. Kevin is inside that circle. Draw a second circle at 10 miles. Search the 10 mile circle first. This concentrates your efforts in the area of highest probability. This may be as simple as he walked into thick underbrush to relieve himself and fell, or as complex as searching out a quite place with a nice view, that is somewhat inaccessible. Do not make the mistake not searching a wash, depression, thicket of briers, etc thinking he did not or could not have gone in there. If the area is very difficult or dangerous, have the SAR teams high angle experts do the looking.