Thermal imaging search was a topic in my AO the last couple of times this summer when hikers went missing here on 2 different occasions. In short, thermal imaging is difficult at best and can be greatly hampered by tree cover, deep valleys, gorges, canyons etc.

There are a couple of ex and active SAR on this forum and I am sure they can go into more detail on thermal imaging search techniques.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock