[gojomo]

Without knowing him or his details specifically, but considering the range of possibilities generically:

A person in such a situation could be the victim of a crime or accident, or of a health episode – physical or mental – that's caused temporary incapacitation. That incapacitation might mean they're still ambulatory, but confused and in need of help, or alternatively trapped/disabled/unconscious somewhere – perhaps even not far from help, if people were just perceptive enough to notice.

Consider, for example, the case of the patient lost and deceased in the stairwell of the SF General Hospital last year: http://www.sfgate.com/bayarea/article/How-S-F-General-lost-t...

A slightly more thorough search, rather than the assumption "she's an adult, she must have wandered off the grounds for her own purposes", might have discovered her unconscious in time to save her life.

Of course, if in any such case the person truly is a competent adult devoutly not wanting to be found, they won't be. But if friends/family are concerned, it's better to trust their particular knowledge.