I still think you're referring to LiDAR (which is not thermal, it's using light and refraction, at least as I understand it). It requires sequential flyovers of incredible detail. It's also used to help build self-driving cars (basically like seeing with really good eyes, the light is bouncing back to the laser sensor in a very directed way).
This Ancient Civilization Was Twice As Big As Medieval England
I posted more links yesterday. It is very expensive. I do think that's what you're thinking of - and it would require specialized software to detect (over time) a human body. What it's detecting in the ruins is the subtle and gradual subsiding of soil as hollow spaces in the ground slowly collapse. In that way, it gives a map of rooms and roads that are now under ground (under other ruins). It's very exciting for archaeologists.
I don't know of any method in which stones and rocks in ruins radiate heat to the surface or any method by which either archaeologists or geologists are able to type/image stones/bones underground using the heat of those objects.
But, to use it to find a body, one would be measuring the subsidence of the body itself. The body would need to be within the initial "photos" and then appear to be reducing in size as time went on. Or if someone dug a grave, that might show up too. But there are other, cheaper ways to probe for graves and the number of pass overs required with the laser tech is unknown and dependent on the rate of soil subsidence in the area mapped. If the Mayans hadn't elevated their roadways (which then collapsed into cake-like layers), this tech would only have worked for their buildings.
I can't find any evidence that it has been used for bodies. And since paleoanthropologists live to find bones and bodies, if it were feasible, I'm sure they are adapting it. No one I've talked to sees how one could use LiDAR (at this point in time) to find bodies.
Really good drone shots, combined with AI, can do it though. This is especially true if there's a high res photograph of the area (which I estimate to be 16-20 square miles for Barbara, at a minimum) to start with. Then, a computer can sense change points in future high res photographs, at different points in time. No lasers needed. Bodies have been found with this method. Bodies have also been found by volunteers studying high res from just one pass over. Some people are super talented at this (it's almost more of a mental gift than a learned skill). It's like those Highlights "find the object" puzzles most of us saw as kids. If, however, the body is under something or obscured from a sky view, it doesn't work so well.
That's why most missing persons are found by human eyes, boots on the ground. I just went to a SAR training (I'm not SAR, but am sometimes involved in training/recruitment) and they said 70% of finding humans (live or not) is human eyes, not dogs or drones. But drones could really help in the Mojave, I think.
I'm still curious about the decomp method you mentioned though - it sounds intriguing (but I'm guessing that the amount of bacteria and insect life is higher in more forested areas, so it would be interesting to know where it's been used).
Citations on LiDAR (used to find the Mayan and Angkor ruins):
https://www.sciencedirect.com/science/article/pii/S0305440310003286
Geospatial revolution and remote sensing LiDAR in Mesoamerican archaeology
Uncovering archaeological landscapes at Angkor using lidar
The reason this was so useful in lowland Maya areas is that this system can "see" through leaves and down to the ground, which has characteristic patterns (created over a period of many centuries) that show signs of civilization beneath (roads, buildings, bridges, etc). Graves can be seen as grave-sized depressions, but the objects need to cause displacement of the topsoil. They're using it now to try and peer down on Stonehenge to see if there are now-hidden stones (holding up topsoil while the remainder subsides).
