HEAT EXHAUSTION is due to hyperthermia and lead to HEAT STROKE, which can be fatal. I have personal experience, too, and my signal symptom is a bad headache, as I am not at all prone to headaches. Pale skin, fair hair with ancestors from the cold north, I am not genetically adapted to live in hot or especially hot and humid environments. I have to retreat to the shade and get wet cloths on me to bring my temp down.
I have experience with it too. And I know first hand that as my body temperature went up, at one point I entered a state of euphoria and was not acting like myself at all. I was driving a truck that had no A/C in the Mojave. My internal body temperature when we got to Needles (family was in A/C car) was a bit over 102. Fortunately, the person driving the other car saw right away that I was not okay. I thought I was okay.
It was 118 in the shade in Needles. I'd been driving in that heat for a couple of hours, used up all my water (and through a misunderstanding, the driver of the other car had taken my bigger water jug. I had the paradoxical chills at one point as well.
Early warning signs include a flushed face,
hyperventilation, headache,
dizziness, nausea, arm tingling, piloerection, paradoxical chilliness, incoordination, and confusion.
Pain may not be present and there may be euphoria.
^
https://www.sciencedirect.com/topics/medicine-and-dentistry/heat-exhaustion
My daughter experienced heat stroke in Havasu City, to the point that she lost consciousness. She too went through the euphoria phase and onward to a very bad headache and other symptoms. So my case was likely heat exhaustion and hers was probably heatstroke.
In Death Valley, a man on a motorcycle experienced heat stroke, lost consciousness while driving, and died from the resulting death. In Grand Canyon, I assisted in a search for a young man from West Virginia who died of heat stroke only 2 miles from safety, having wandered off the Grandview Trail in search of (non-existent) shade. His death resulted in the park deciding to put bigger signs with better instructions about heat at every trailhead.
So any time someone goes missing in July in the Mojave, I can't help but think heat could be a factor - even moderate heat exhaustion can cause confusion, euphoria, paradoxical chilling, etc.
This scenario would presume that the victims were out of their A/C truck for more than 15-20 minutes and in full sun (which they would be near Harper Flat, no trees, no large boulders, no shade at all). I'm sure it's a scenario that has been considered by LE in their search, and it's very hard to diagnose from a skeleton (if not impossible). In the Grand Canyon case I mentioned, the determination was made due to the man's having run out of water and having curled up under a small pinyon. It was about 110-112 along the trail that day. In that case, dehydration did play a role. In the motorcycle guy's case, it appeared he still had water, but had been driving (in a helmet) for more than an hour in 117-118F heat. He was in his 50's. Medics were able to get his core body temperature soon after the wreck.
But anyway, it's hard to diagnose from a skeleton.
IMO.