My DH is involved in search and rescue, so we had a talk this morning about how they approach a search for someone missing. It is complicated, as so many factors which start with the age and fitness of the person (eg a person with dementia will tend to take a path and keep going along it, a frail unfit person will probably choose an easier or downhill path at a crossroads etc) , then they think about the emotional state of the person, and their general personality and habits. Then you start with the last known sighting of the person and there are techniques for working out how to radiate outwards, like spokes in a wheel, thinking of possible destinations , with the spokes being paths, roads or trails.
There are are so many different things to consider depending on landscape and climate too.
I very much hope he is found this afternoon.